subsys: fs: fix generic storage partition selection

FS_FLASH_MAP_STORAGE keyword enables the storage partition,
but it was depend on flash_map module which is unused by
NFFS. This patch makes it independent thanks
to it is possible to enable the storage partition
without flash_map module.

FS_FLASH_MAP_STORAGE was renamed to
CONFIG_FS_FLASH_STORAGE_PARTITION
as it is independent for flash_map.

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