west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  346f7374ff4467e40b5594658f8ac67a5e9813c9

Brings following Zephyr relevant fixes:
  - 2aa6da15 boot: Add flash area ID/device ID retrieval hooks
  - b3ed5cc6 boot: New boot_go hook
  - dbbcb78b boot/zephyr: Fix SINGLE_APPLICATION_SLOT_RAM_LOAD
    file inclusion
  - 33094fc5 boot: bootutil: loader: Fix issue with stuck revert

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