Documentation clarifications

After running though the documentation a few things could be clarified.

Document how to confirm PHiLIP flash was successful (by blinking LED)
Reference how to get portnames on linux (/dev/tty*)
Add troubleshooting if DUT portname is not default
make sure you run from test folder or use -C tests/per...
Add troubleshooting (tell people to double check pinout if only a few tests fail, point out you need to point to a riot version)
Clarify example use of RIOTBASE

RIOT-OS / RobotFW-tests