stm32: Make up_progmem thread safe

Writing to a flash sector while starting the erase of other sector
have a undefined behavior so lets add a semaphore and syncronize
access to Flash registers.

But for the semaphore to work it needs to be initialized so each
board needs call stm32_flash_initialize() on initialization, so
to avoid runtime problems it is only using semaphore and making
it thread safe if initialized, after all boards starts to call
stm32_flash_initialize() we can remove the boolean and the check.