manifest: Mcuboot update

Mcuboot was updated to the version synchronized with
the upstream 5b7ed6a which introduces:
- Added LED support for signaling enter to the serial recovery mode
- Added boot delay/debonuce of serial detect pin.
- Fixed compilation warnings with ZEPHYR_LOG_MODE_MINIMAL
- Added optional check which prevents attempting to boot image that
has been build for different ROM address than a slot it currently
resides in. Check is enabled if image was signed with
IMAGE_F_ROM_FIXED flag.

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