west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  4fe28b3cf6d30c3e57d6a4f7ee3dba8617aa9a0a

Brings following Zephyr relevant fixes:
 - 4fe28b3 Update zephyr version files for 2.0.0-rc1
 - 6a6de4b scripts: imgtool: update to 2.0.0-rc1 release

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