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>