loader: added hooks for override mcuboot image access operation

Added hook for read image header.

Added hook which is for inject an image check implementation which can
be used instead of boot_image_check() internal implementation on a primary
slot.
This is usefully especially when primary has to be checked by other entity
on MCUboot's demand.

Added hook which allow to implement image update process
differently.

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