dist/tools/testrunner: Make started wait time configurable
dist/tools/testrunner: Make started wait time configurable

The wait before board is started is 3 by default but can be overwritten by
setting 'TESTRUNNER_START_WAIT' environment variable.

gebart/RIOT,RIOT-OS/RIOT,roberthartung/RIOT,authmillenon/RIOT,immesys/RiSyn,kYc0o/RIOT,kaspar030/RIOT,neiljay/RIOT,mtausig/RIOT,mfrey/RIOT,LudwigOrtmann/RIOT,A-Paul/RIOT,yogo1212/RIOT,biboc/RIOT,immesys/RiSyn,miri64/RIOT,cladmi/RIOT,toonst/RIOT,neiljay/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,mfrey/RIOT,gebart/RIOT,RIOT-OS/RIOT,aeneby/RIOT,ant9000/RIOT,kaspar030/RIOT,josephnoir/RIOT,aeneby/RIOT,LudwigKnuepfer/RIOT,rfuentess/RIOT,basilfx/RIOT,Josar/RIOT,x3ro/RIOT,josephnoir/RIOT,OTAkeys/RIOT,gebart/RIOT,kYc0o/RIOT,Josar/RIOT,toonst/RIOT,ant9000/RIOT,basilfx/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,aeneby/RIOT,mfrey/RIOT,OlegHahm/RIOT,LudwigKnuepfer/RIOT,kaspar030/RIOT,x3ro/RIOT,jasonatran/RIOT,ant9000/RIOT,BytesGalore/RIOT,kbumsik/RIOT,authmillenon/RIOT,roberthartung/RIOT,jasonatran/RIOT,kbumsik/RIOT,rfuentess/RIOT,biboc/RIOT,roberthartung/RIOT,kYc0o/RIOT,lazytech-org/RIOT,authmillenon/RIOT,yogo1212/RIOT,jasonatran/RIOT,roberthartung/RIOT,neiljay/RIOT,mfrey/RIOT,neiljay/RIOT,BytesGalore/RIOT,x3ro/RIOT,miri64/RIOT,avmelnikoff/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,biboc/RIOT,miri64/RIOT,LudwigKnuepfer/RIOT,BytesGalore/RIOT,rfuentess/RIOT,mtausig/RIOT,basilfx/RIOT,authmillenon/RIOT,biboc/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,ant9000/RIOT,smlng/RIOT,LudwigOrtmann/RIOT,josephnoir/RIOT,lazytech-org/RIOT,Josar/RIOT,kbumsik/RIOT,authmillenon/RIOT,kbumsik/RIOT,immesys/RiSyn,LudwigOrtmann/RIOT,yogo1212/RIOT,OTAkeys/RIOT,smlng/RIOT,LudwigKnuepfer/RIOT,immesys/RiSyn,neiljay/RIOT,x3ro/RIOT,RIOT-OS/RIOT,biboc/RIOT,cladmi/RIOT,OlegHahm/RIOT,mfrey/RIOT,yogo1212/RIOT,toonst/RIOT,lazytech-org/RIOT,miri64/RIOT,aeneby/RIOT,gebart/RIOT,kbumsik/RIOT,immesys/RiSyn,mtausig/RIOT,cladmi/RIOT,kYc0o/RIOT,immesys/RiSyn,Josar/RIOT,cladmi/RIOT,mtausig/RIOT,basilfx/RIOT,aeneby/RIOT,josephnoir/RIOT,ant9000/RIOT,roberthartung/RIOT,rfuentess/RIOT,josephnoir/RIOT,smlng/RIOT,Josar/RIOT,basilfx/RIOT,gebart/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,miri64/RIOT,yogo1212/RIOT,LudwigOrtmann/RIOT,kaspar030/RIOT,toonst/RIOT,A-Paul/RIOT,smlng/RIOT,lazytech-org/RIOT,LudwigKnuepfer/RIOT,smlng/RIOT,kYc0o/RIOT,jasonatran/RIOT,rfuentess/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,toonst/RIOT,LudwigOrtmann/RIOT,kaspar030/RIOT,OlegHahm/RIOT,mtausig/RIOT,authmillenon/RIOT,cladmi/RIOT,jasonatran/RIOT,A-Paul/RIOT,LudwigOrtmann/RIOT,BytesGalore/RIOT,avmelnikoff/RIOT,avmelnikoff/RIOT,x3ro/RIOT,avmelnikoff/RIOT