west.yaml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  9bf7ce8c5fe8152836a6e00bd4444153bd950342

Brings following Zephyr relevant fixes:
 - 9bf7ce8 zephyr: Fix build for non-arm archs
 - e188dbb zephyr: Fix boot serial extensions

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
1 file changed