shell: let readline return an error on empty line so prompt gets print again
shell: let readline return an error on empty line so prompt gets print again

jremmert-phytec-iot/RIOT,DipSwitch/RIOT,PSHIVANI/Riot-Code,EmuxEvans/RIOT,centurysys/RIOT,mfrey/RIOT,aeneby/RIOT,bartfaizoltan/RIOT,binarylemon/RIOT,binarylemon/RIOT,jbeyerstedt/RIOT-OTA-update,herrfz/RIOT,immesys/RiSyn,arvindpdmn/RIOT,Darredevil/RIOT,ntrtrung/RIOT,malosek/RIOT,altairpearl/RIOT,stevenj/RIOT,RBartz/RIOT,neumodisch/RIOT,msolters/RIOT,x3ro/RIOT,malosek/RIOT,kaleb-himes/RIOT,miri64/RIOT,plushvoxel/RIOT,patkan/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,smlng/RIOT,koenning/RIOT,Osblouf/RIOT,MarkXYang/RIOT,LudwigOrtmann/RIOT,RIOT-OS/RIOT,patkan/RIOT,ThanhVic/RIOT,MohmadAyman/RIOT,mfrey/RIOT,kerneltask/RIOT,dailab/RIOT,sumanpanchal/RIOT,openkosmosorg/RIOT,Ell-i/RIOT,kushalsingh007/RIOT,MarkXYang/RIOT,dailab/RIOT,authmillenon/RIOT,khhhh/RIOT,jremmert-phytec-iot/RIOT,dailab/RIOT,sumanpanchal/RIOT,koenning/RIOT,dhruvvyas90/RIOT,jbeyerstedt/RIOT-OTA-update,d00616/RIOT,MarkXYang/RIOT,Lexandro92/RIOT-CoAP,kb2ma/RIOT,daniel-k/RIOT,miri64/RIOT,MarkXYang/RIOT,l3nko/RIOT,JensErdmann/RIOT,robixnai/RIOT,neiljay/RIOT,wentaoshang/RIOT,beurdouche/RIOT,khhhh/RIOT,openkosmosorg/RIOT,LudwigKnuepfer/RIOT,thiagohd/RIOT,watr-li/RIOT,malosek/RIOT,FrancescoErmini/RIOT,DipSwitch/RIOT,TobiasFredersdorf/RIOT,kaspar030/RIOT,attdona/RIOT,jferreir/RIOT,mfrey/RIOT,RBartz/RIOT,thiagohd/RIOT,immesys/RiSyn,MonsterCode8000/RIOT,avmelnikoff/RIOT,asanka-code/RIOT,centurysys/RIOT,OlegHahm/RIOT,malosek/RIOT,ThanhVic/RIOT,patkan/RIOT,syin2/RIOT,jferreir/RIOT,MonsterCode8000/RIOT,kushalsingh007/RIOT,mziegert/RIOT,binarylemon/RIOT,Lexandro92/RIOT-CoAP,wentaoshang/RIOT,neumodisch/RIOT,jasonatran/RIOT,kbumsik/RIOT,alignan/RIOT,zhuoshuguo/RIOT,gbarnett/RIOT,biboc/RIOT,syin2/RIOT,kerneltask/RIOT,TobiasFredersdorf/RIOT,rajma996/RIOT,hamilton-mote/RIOT-OS,mziegert/RIOT,Hyungsin/RIOT-OS,ThanhVic/RIOT,haoyangyu/RIOT,rakendrathapa/RIOT,rajma996/RIOT,FrancescoErmini/RIOT,rousselk/RIOT,backenklee/RIOT,rousselk/RIOT,OlegHahm/RIOT,alex1818/RIOT,luciotorre/RIOT,jfischer-phytec-iot/RIOT,JensErdmann/RIOT,RIOT-OS/RIOT,altairpearl/RIOT,Darredevil/RIOT,DipSwitch/RIOT,Darredevil/RIOT,DipSwitch/RIOT,sumanpanchal/RIOT,daniel-k/RIOT,ximus/RIOT,immesys/RiSyn,rfuentess/RIOT,katezilla/RIOT,cladmi/RIOT,mtausig/RIOT,Yonezawa-T2/RIOT,authmillenon/RIOT,Josar/RIOT,Darredevil/RIOT,BytesGalore/RIOT,PSHIVANI/Riot-Code,sumanpanchal/RIOT,MohmadAyman/RIOT,basilfx/RIOT,kaleb-himes/RIOT,kaleb-himes/RIOT,lebrush/RIOT,jasonatran/RIOT,beurdouche/RIOT,robixnai/RIOT,ks156/RIOT,Josar/RIOT,aeneby/RIOT,ant9000/RIOT,luciotorre/RIOT,gbarnett/RIOT,rousselk/RIOT,brettswann/RIOT,dkm/RIOT,arvindpdmn/RIOT,RubikonAlpha/RIOT,kaleb-himes/RIOT,EmuxEvans/RIOT,dhruvvyas90/RIOT,dhruvvyas90/RIOT,RubikonAlpha/RIOT,openkosmosorg/RIOT,binarylemon/RIOT,daniel-k/RIOT,herrfz/RIOT,gbarnett/RIOT,Ell-i/RIOT,wentaoshang/RIOT,lebrush/RIOT,mziegert/RIOT,JensErdmann/RIOT,Osblouf/RIOT,sumanpanchal/RIOT,ant9000/RIOT,EmuxEvans/RIOT,BytesGalore/RIOT,thomaseichinger/RIOT,PSHIVANI/Riot-Code,daniel-k/RIOT,Josar/RIOT,josephnoir/RIOT,Yonezawa-T2/RIOT,mtausig/RIOT,asanka-code/RIOT,FrancescoErmini/RIOT,neumodisch/RIOT,Darredevil/RIOT,DipSwitch/RIOT,Osblouf/RIOT,luciotorre/RIOT,automote/RIOT,MarkXYang/RIOT,rfuentess/RIOT,dkm/RIOT,mfrey/RIOT,yogo1212/RIOT,gebart/RIOT,cladmi/RIOT,EmuxEvans/RIOT,kushalsingh007/RIOT,watr-li/RIOT,abkam07/RIOT,OTAkeys/RIOT,dhruvvyas90/RIOT,attdona/RIOT,basilfx/RIOT,yogo1212/RIOT,tfar/RIOT,katezilla/RIOT,Darredevil/RIOT,syin2/RIOT,kushalsingh007/RIOT,lebrush/RIOT,ThanhVic/RIOT,aeneby/RIOT,luciotorre/RIOT,chris-wood/RIOT,d00616/RIOT,kaspar030/RIOT,neumodisch/RIOT,aeneby/RIOT,watr-li/RIOT,ntrtrung/RIOT,backenklee/RIOT,hamilton-mote/RIOT-OS,rfuentess/RIOT,biboc/RIOT,automote/RIOT,arvindpdmn/RIOT,BytesGalore/RIOT,robixnai/RIOT,LudwigKnuepfer/RIOT,rakendrathapa/RIOT,marcosalm/RIOT,Josar/RIOT,ant9000/RIOT,jasonatran/RIOT,d00616/RIOT,jbeyerstedt/RIOT-OTA-update,adjih/RIOT,katezilla/RIOT,gebart/RIOT,roberthartung/RIOT,immesys/RiSyn,jfischer-phytec-iot/RIOT,gbarnett/RIOT,chris-wood/RIOT,ks156/RIOT,changbiao/RIOT,A-Paul/RIOT,haoyangyu/RIOT,gebart/RIOT,rajma996/RIOT,alignan/RIOT,Hyungsin/RIOT-OS,RIOT-OS/RIOT,toonst/RIOT,adjih/RIOT,ximus/RIOT,lazytech-org/RIOT,latsku/RIOT,ntrtrung/RIOT,alex1818/RIOT,marcosalm/RIOT,Lexandro92/RIOT-CoAP,latsku/RIOT,FrancescoErmini/RIOT,ant9000/RIOT,attdona/RIOT,dhruvvyas90/RIOT,brettswann/RIOT,wentaoshang/RIOT,FrancescoErmini/RIOT,arvindpdmn/RIOT,toonst/RIOT,abp719/RIOT,authmillenon/RIOT,JensErdmann/RIOT,ks156/RIOT,rakendrathapa/RIOT,kYc0o/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,khhhh/RIOT,smlng/RIOT,lazytech-org/RIOT,l3nko/RIOT,lazytech-org/RIOT,Osblouf/RIOT,alex1818/RIOT,gebart/RIOT,khhhh/RIOT,beurdouche/RIOT,haoyangyu/RIOT,rajma996/RIOT,ximus/RIOT,LudwigOrtmann/RIOT,LudwigKnuepfer/RIOT,TobiasFredersdorf/RIOT,msolters/RIOT,OTAkeys/RIOT,jferreir/RIOT,watr-li/RIOT,stevenj/RIOT,alex1818/RIOT,biboc/RIOT,gbarnett/RIOT,RBartz/RIOT,tdautc19841202/RIOT,dkm/RIOT,RIOT-OS/RIOT,tdautc19841202/RIOT,altairpearl/RIOT,lazytech-org/RIOT,rfuentess/RIOT,jfischer-phytec-iot/RIOT,RubikonAlpha/RIOT,Osblouf/RIOT,beurdouche/RIOT,changbiao/RIOT,adjih/RIOT,thiagohd/RIOT,openkosmosorg/RIOT,rakendrathapa/RIOT,adrianghc/RIOT,altairpearl/RIOT,adrianghc/RIOT,l3nko/RIOT,thiagohd/RIOT,MohmadAyman/RIOT,immesys/RiSyn,mfrey/RIOT,centurysys/RIOT,smlng/RIOT,avmelnikoff/RIOT,ntrtrung/RIOT,adrianghc/RIOT,tdautc19841202/RIOT,brettswann/RIOT,OTAkeys/RIOT,avmelnikoff/RIOT,zhuoshuguo/RIOT,jasonatran/RIOT,lebrush/RIOT,MohmadAyman/RIOT,koenning/RIOT,haoyangyu/RIOT,miri64/RIOT,centurysys/RIOT,jferreir/RIOT,sumanpanchal/RIOT,patkan/RIOT,ximus/RIOT,roberthartung/RIOT,rakendrathapa/RIOT,smlng/RIOT,koenning/RIOT,thiagohd/RIOT,changbiao/RIOT,abkam07/RIOT,attdona/RIOT,ximus/RIOT,yogo1212/RIOT,kbumsik/RIOT,x3ro/RIOT,roberthartung/RIOT,kYc0o/RIOT,MohmadAyman/RIOT,patkan/RIOT,A-Paul/RIOT,msolters/RIOT,x3ro/RIOT,bartfaizoltan/RIOT,LudwigOrtmann/RIOT,OlegHahm/RIOT,RubikonAlpha/RIOT,tdautc19841202/RIOT,alex1818/RIOT,latsku/RIOT,stevenj/RIOT,MonsterCode8000/RIOT,rajma996/RIOT,neumodisch/RIOT,smlng/RIOT,OTAkeys/RIOT,robixnai/RIOT,syin2/RIOT,jremmert-phytec-iot/RIOT,d00616/RIOT,jferreir/RIOT,bartfaizoltan/RIOT,aeneby/RIOT,kb2ma/RIOT,brettswann/RIOT,daniel-k/RIOT,chris-wood/RIOT,marcosalm/RIOT,yogo1212/RIOT,robixnai/RIOT,malosek/RIOT,LudwigKnuepfer/RIOT,Yonezawa-T2/RIOT,shady33/RIOT,changbiao/RIOT,attdona/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,herrfz/RIOT,herrfz/RIOT,stevenj/RIOT,gebart/RIOT,kbumsik/RIOT,jasonatran/RIOT,A-Paul/RIOT,gautric/RIOT,kb2ma/RIOT,abp719/RIOT,thomaseichinger/RIOT,roberthartung/RIOT,BytesGalore/RIOT,alignan/RIOT,marcosalm/RIOT,alex1818/RIOT,koenning/RIOT,EmuxEvans/RIOT,yogo1212/RIOT,BytesGalore/RIOT,ntrtrung/RIOT,OlegHahm/RIOT,abkam07/RIOT,thomaseichinger/RIOT,ximus/RIOT,abp719/RIOT,miri64/RIOT,shady33/RIOT,toonst/RIOT,adrianghc/RIOT,binarylemon/RIOT,ks156/RIOT,marcosalm/RIOT,gautric/RIOT,chris-wood/RIOT,thiagohd/RIOT,luciotorre/RIOT,adjih/RIOT,automote/RIOT,luciotorre/RIOT,ThanhVic/RIOT,l3nko/RIOT,neiljay/RIOT,basilfx/RIOT,DipSwitch/RIOT,lebrush/RIOT,dhruvvyas90/RIOT,basilfx/RIOT,RIOT-OS/RIOT,dkm/RIOT,patkan/RIOT,attdona/RIOT,yogo1212/RIOT,josephnoir/RIOT,centurysys/RIOT,PSHIVANI/Riot-Code,MonsterCode8000/RIOT,lebrush/RIOT,brettswann/RIOT,mziegert/RIOT,alignan/RIOT,ntrtrung/RIOT,l3nko/RIOT,abp719/RIOT,hamilton-mote/RIOT-OS,chris-wood/RIOT,backenklee/RIOT,l3nko/RIOT,JensErdmann/RIOT,arvindpdmn/RIOT,koenning/RIOT,tfar/RIOT,Hyungsin/RIOT-OS,asanka-code/RIOT,altairpearl/RIOT,jremmert-phytec-iot/RIOT,alignan/RIOT,beurdouche/RIOT,openkosmosorg/RIOT,biboc/RIOT,arvindpdmn/RIOT,plushvoxel/RIOT,josephnoir/RIOT,watr-li/RIOT,herrfz/RIOT,jremmert-phytec-iot/RIOT,latsku/RIOT,msolters/RIOT,MohmadAyman/RIOT,automote/RIOT,mtausig/RIOT,avmelnikoff/RIOT,wentaoshang/RIOT,khhhh/RIOT,immesys/RiSyn,OlegHahm/RIOT,neumodisch/RIOT,kerneltask/RIOT,mtausig/RIOT,Lexandro92/RIOT-CoAP,d00616/RIOT,rousselk/RIOT,RubikonAlpha/RIOT,A-Paul/RIOT,thomaseichinger/RIOT,kbumsik/RIOT,tfar/RIOT,josephnoir/RIOT,bartfaizoltan/RIOT,RBartz/RIOT,cladmi/RIOT,biboc/RIOT,asanka-code/RIOT,automote/RIOT,A-Paul/RIOT,LudwigOrtmann/RIOT,bartfaizoltan/RIOT,d00616/RIOT,chris-wood/RIOT,toonst/RIOT,herrfz/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,adrianghc/RIOT,kaspar030/RIOT,tdautc19841202/RIOT,shady33/RIOT,x3ro/RIOT,abkam07/RIOT,roberthartung/RIOT,RBartz/RIOT,katezilla/RIOT,avmelnikoff/RIOT,neiljay/RIOT,tfar/RIOT,kb2ma/RIOT,kYc0o/RIOT,haoyangyu/RIOT,shady33/RIOT,zhuoshuguo/RIOT,zhuoshuguo/RIOT,shady33/RIOT,hamilton-mote/RIOT-OS,stevenj/RIOT,ant9000/RIOT,jfischer-phytec-iot/RIOT,rousselk/RIOT,gautric/RIOT,jremmert-phytec-iot/RIOT,katezilla/RIOT,EmuxEvans/RIOT,MonsterCode8000/RIOT,kaspar030/RIOT,plushvoxel/RIOT,ThanhVic/RIOT,abkam07/RIOT,openkosmosorg/RIOT,centurysys/RIOT,msolters/RIOT,Josar/RIOT,rakendrathapa/RIOT,kerneltask/RIOT,automote/RIOT,OTAkeys/RIOT,LudwigKnuepfer/RIOT,stevenj/RIOT,daniel-k/RIOT,Yonezawa-T2/RIOT,Ell-i/RIOT,thomaseichinger/RIOT,mtausig/RIOT,watr-li/RIOT,tfar/RIOT,shady33/RIOT,Ell-i/RIOT,kb2ma/RIOT,latsku/RIOT,Osblouf/RIOT,Ell-i/RIOT,kushalsingh007/RIOT,FrancescoErmini/RIOT,MarkXYang/RIOT,miri64/RIOT,dailab/RIOT,toonst/RIOT,Hyungsin/RIOT-OS,RubikonAlpha/RIOT,cladmi/RIOT,Yonezawa-T2/RIOT,asanka-code/RIOT,jbeyerstedt/RIOT-OTA-update,wentaoshang/RIOT,robixnai/RIOT,mziegert/RIOT,dailab/RIOT,kbumsik/RIOT,kYc0o/RIOT,rousselk/RIOT,authmillenon/RIOT,rajma996/RIOT,latsku/RIOT,neiljay/RIOT,haoyangyu/RIOT,gautric/RIOT,authmillenon/RIOT,jfischer-phytec-iot/RIOT,zhuoshuguo/RIOT,backenklee/RIOT,tdautc19841202/RIOT,syin2/RIOT,backenklee/RIOT,gbarnett/RIOT,Lexandro92/RIOT-CoAP,changbiao/RIOT,PSHIVANI/Riot-Code,Hyungsin/RIOT-OS,kerneltask/RIOT,asanka-code/RIOT,plushvoxel/RIOT,TobiasFredersdorf/RIOT,neiljay/RIOT,changbiao/RIOT,hamilton-mote/RIOT-OS,kaspar030/RIOT,plushvoxel/RIOT,Lexandro92/RIOT-CoAP,LudwigOrtmann/RIOT,cladmi/RIOT,altairpearl/RIOT,RBartz/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,bartfaizoltan/RIOT,adjih/RIOT,malosek/RIOT,mziegert/RIOT,x3ro/RIOT,basilfx/RIOT,dkm/RIOT,Yonezawa-T2/RIOT,abp719/RIOT,TobiasFredersdorf/RIOT,LudwigOrtmann/RIOT,JensErdmann/RIOT,abp719/RIOT,ks156/RIOT,kushalsingh007/RIOT,josephnoir/RIOT,authmillenon/RIOT,binarylemon/RIOT,brettswann/RIOT,kYc0o/RIOT,marcosalm/RIOT,PSHIVANI/Riot-Code,msolters/RIOT,kaleb-himes/RIOT,gautric/RIOT,jferreir/RIOT,rfuentess/RIOT,lazytech-org/RIOT,jbeyerstedt/RIOT-OTA-update,abkam07/RIOT,zhuoshuguo/RIOT,khhhh/RIOT,MonsterCode8000/RIOT