subsys: storage: flash_map: enable only if any flash driver is enable

It was possible to have enable flash module while no flash driver
implementation was enabled. This cause coverity issues and unnecessary
initialization call.

This pat introduce FLASH_HAS_DRIVER_ENABLED Kconfig keyword which is
selected once any flash driver is enabled. flash_map switch its
dependency to this keyword.


Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>