sys/shell: fix build when SHELL_NO_ECHO is defined
sys/shell: fix build when SHELL_NO_ECHO is defined

When SHELL_NO_ECHO was defined and not SHELL_NO_PROMPT build was broken
because _putchar was not defined.
This define _putchar when one of SHELL_NO_PROMPT or SHELL_NO_ECHO is not
defined.

neiljay/RIOT,jfischer-phytec-iot/RIOT,miri64/RIOT,jfischer-phytec-iot/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,immesys/RiSyn,thomaseichinger/RIOT,BytesGalore/RIOT,thomaseichinger/RIOT,authmillenon/RIOT,x3ro/RIOT,LudwigKnuepfer/RIOT,ant9000/RIOT,LudwigOrtmann/RIOT,neiljay/RIOT,mfrey/RIOT,LudwigKnuepfer/RIOT,cladmi/RIOT,LudwigOrtmann/RIOT,gebart/RIOT,neumodisch/RIOT,OlegHahm/RIOT,gebart/RIOT,roberthartung/RIOT,authmillenon/RIOT,dkm/RIOT,aeneby/RIOT,aeneby/RIOT,yogo1212/RIOT,gebart/RIOT,dailab/RIOT,adjih/RIOT,Josar/RIOT,adrianghc/RIOT,jasonatran/RIOT,biboc/RIOT,OlegHahm/RIOT,basilfx/RIOT,Ell-i/RIOT,toonst/RIOT,A-Paul/RIOT,basilfx/RIOT,josephnoir/RIOT,hamilton-mote/RIOT-OS,cladmi/RIOT,immesys/RiSyn,avmelnikoff/RIOT,avmelnikoff/RIOT,lazytech-org/RIOT,ant9000/RIOT,kaspar030/RIOT,dkm/RIOT,LudwigKnuepfer/RIOT,toonst/RIOT,biboc/RIOT,jasonatran/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,biboc/RIOT,toonst/RIOT,kbumsik/RIOT,kbumsik/RIOT,A-Paul/RIOT,adjih/RIOT,yogo1212/RIOT,adjih/RIOT,x3ro/RIOT,OTAkeys/RIOT,Ell-i/RIOT,ks156/RIOT,kerneltask/RIOT,kerneltask/RIOT,Ell-i/RIOT,gebart/RIOT,LudwigOrtmann/RIOT,yogo1212/RIOT,hamilton-mote/RIOT-OS,Hyungsin/RIOT-OS,smlng/RIOT,cladmi/RIOT,beurdouche/RIOT,josephnoir/RIOT,kbumsik/RIOT,miri64/RIOT,plushvoxel/RIOT,neumodisch/RIOT,smlng/RIOT,A-Paul/RIOT,josephnoir/RIOT,yogo1212/RIOT,dkm/RIOT,OTAkeys/RIOT,hamilton-mote/RIOT-OS,x3ro/RIOT,OTAkeys/RIOT,ant9000/RIOT,gautric/RIOT,beurdouche/RIOT,Josar/RIOT,plushvoxel/RIOT,mfrey/RIOT,plushvoxel/RIOT,kaleb-himes/RIOT,thomaseichinger/RIOT,cladmi/RIOT,kaspar030/RIOT,yogo1212/RIOT,neiljay/RIOT,thomaseichinger/RIOT,thomaseichinger/RIOT,dailab/RIOT,OlegHahm/RIOT,josephnoir/RIOT,lazytech-org/RIOT,LudwigOrtmann/RIOT,kbumsik/RIOT,authmillenon/RIOT,roberthartung/RIOT,OTAkeys/RIOT,neumodisch/RIOT,ks156/RIOT,mfrey/RIOT,dkm/RIOT,LudwigKnuepfer/RIOT,authmillenon/RIOT,kYc0o/RIOT,mfrey/RIOT,kYc0o/RIOT,immesys/RiSyn,RIOT-OS/RIOT,roberthartung/RIOT,biboc/RIOT,RIOT-OS/RIOT,Hyungsin/RIOT-OS,immesys/RiSyn,aeneby/RIOT,roberthartung/RIOT,Ell-i/RIOT,OlegHahm/RIOT,neiljay/RIOT,syin2/RIOT,hamilton-mote/RIOT-OS,beurdouche/RIOT,x3ro/RIOT,BytesGalore/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,LudwigOrtmann/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,toonst/RIOT,LudwigOrtmann/RIOT,rfuentess/RIOT,BytesGalore/RIOT,adrianghc/RIOT,neiljay/RIOT,kYc0o/RIOT,plushvoxel/RIOT,kerneltask/RIOT,Josar/RIOT,jfischer-phytec-iot/RIOT,basilfx/RIOT,gautric/RIOT,aeneby/RIOT,neumodisch/RIOT,ant9000/RIOT,adrianghc/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,immesys/RiSyn,ks156/RIOT,hamilton-mote/RIOT-OS,x3ro/RIOT,authmillenon/RIOT,josephnoir/RIOT,BytesGalore/RIOT,basilfx/RIOT,adjih/RIOT,gebart/RIOT,gautric/RIOT,A-Paul/RIOT,smlng/RIOT,avmelnikoff/RIOT,kaleb-himes/RIOT,jasonatran/RIOT,syin2/RIOT,aeneby/RIOT,biboc/RIOT,beurdouche/RIOT,Josar/RIOT,miri64/RIOT,gautric/RIOT,avmelnikoff/RIOT,jfischer-phytec-iot/RIOT,toonst/RIOT,jasonatran/RIOT,lazytech-org/RIOT,mfrey/RIOT,kaleb-himes/RIOT,kerneltask/RIOT,basilfx/RIOT,smlng/RIOT,plushvoxel/RIOT,kerneltask/RIOT,rfuentess/RIOT,Hyungsin/RIOT-OS,yogo1212/RIOT,neumodisch/RIOT,miri64/RIOT,lazytech-org/RIOT,gautric/RIOT,jfischer-phytec-iot/RIOT,A-Paul/RIOT,mtausig/RIOT,neumodisch/RIOT,cladmi/RIOT,kaspar030/RIOT,kaleb-himes/RIOT,mtausig/RIOT,OlegHahm/RIOT,Hyungsin/RIOT-OS,roberthartung/RIOT,kYc0o/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,rfuentess/RIOT,immesys/RiSyn,RIOT-OS/RIOT,ks156/RIOT,Ell-i/RIOT,adjih/RIOT,dailab/RIOT,syin2/RIOT,syin2/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,syin2/RIOT,dailab/RIOT,beurdouche/RIOT,adrianghc/RIOT,mtausig/RIOT,RIOT-OS/RIOT,mtausig/RIOT,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,adrianghc/RIOT,LudwigKnuepfer/RIOT,BytesGalore/RIOT,dailab/RIOT,kbumsik/RIOT,dkm/RIOT,kaspar030/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,kaleb-himes/RIOT,smlng/RIOT,Hyungsin/RIOT-OS,rfuentess/RIOT,ks156/RIOT,Josar/RIOT,mtausig/RIOT,lazytech-org/RIOT