zephyr: support the hook file by MCUBOOT_BOOTUTIL library

This patch make possible MCUBOOT_BOOTUTIL to integrate the
hook file on their own. This is intended to support hook while
the library is just part of the application.
The hooks file for the MCUboot build is now included in the same manner.

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