It is faster to operate directly on flash_area pointer instead
of fetch it all the time using the fcb flash area id.
Also as f_area_id was needed only for get appropriate flash_area
pointer, so it is better to pass it only while initialization
the fcb and not store it in fcb instance data at all.
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
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