[DISCO_F334C8] new target incl. rtos support

New target as a copy of nucleo_f334 with changes/additions in pin and
rtos definitions.
[BUILD_TRAVIS.py] additional line for disco_f334

Adding a line for disco_f334 target and adding the rtos lib to
nucleo_f334 target.
Merge pull request #631 from ohagendorf/target_disco_f334c8

Platform: DISCO_F334C8 addition