fs: littlefs: add ability to build little fs without flash map

LittleFS may be used not only for Flash devices, we can use it for
block devices too.

Add possibility of building LittleFS for case when
CONFIG_FLASH_MAP and CONFIG_FLASH_PAGE_LAYOUT are disabled.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>