[export_test.py][STM32F4]

- changed Target name STM32F407 to STM32F4xx
- with the original name the output of export_test.py for this target
for GCC_ARM was bricked
- perhaps, the reason is the new structure after addition of target
arch_max some days ago?
- now all three targets can be exported and compiled
- but target stm32f4xx can't use the given example rtos/mbed/basic
(target doesn't support rtos)
- now the test project used by export_test.py can be configured:
rtos/mbed/basic or mbed/blinky

- NUCLEO_F401RE and NUCLEO_F411RE can be exported to GCC_ARM