[DISCO/NUCLEO_F3xx] solving RTOS problem

- Changing original STM Cube Driver to call _start instead of main to initialise the rtos when using it. 
- Adding targets (except DISCO_F303VC) to tests.py - to be able to run RTOS_x tests
- a minor bug fix for DISCO_F334: had wrong STDIO_UART_TX/RX pin settings

Nearly all tests are OK. Except for DISCO_F334C8 and NUCLEO_F334R8 the test RTOS_3 (Semaphore resource lock)  and for NUCLEO_F302R8 the test RTOS_7 (timer) failed.
DISCO_F303VC MBED_BLINKY and RTOS_1 are OK.