Moving stm32 f2/f4 flash code into the common area.
**** WARNING **** This change may break your code!!!

Change flash_erase_sector (f2/f4) to take numerical sector argument.

As opposed to using one of the defines in the header, this is more convenient when programatically deciding which sectors to erase.
Add higher level flash_program function for f2/f4 to write a buffer into flash.