[DISCO_F4xx][NUCLEO_F4xx][MDOT_F4] solving RTOS problem

- Changing original STM Cube Driver to call _start instead of main to
initialise the rtos when using it. Without using rtos the behavior is
the same as before.
- Adding DISCO_F429 to rtos
- Adding targets to RTOS_xx tests.
- All tests are OK. Tested with Nucleo and Disco boards. Not tested with
MDOT_F4 but that uses the same hal like nucleo_f411.
Merge pull request #793 from ohagendorf/STMF4xxx_rtos

DISCO_F4xx, NUCLEO_F4xx, MDOT_F4 - fix RTOS problem (init seq)