We will reserve BOARD_FLASH_SIZE for the internal flash on stm32 flash processors, use HAL_PROGRAM_SIZE_LIMIT_KB in the general code base.
Notable change here is that boards with external flash will start to get features only available with more than 2MB of program storage
this doesn't actually allow you to specify a default, despite the name
and a couple of hwdefs trying to use it that way.
This would have stopped being useful when we did the
compass-prority-table stuff - or before.
