subsys: storage: flash_map: devices bindings fetch optimalization

Moved fetch of flash device bindings to early initialization of the
application.
Device bindings are constant while the application is running so
it is better to fetch it at startup, and not every time flash_map
procedures are called.


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