west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  3430520d7080a8ac3e4b325cfde0ef18837209af

Brings following Zephyr relevant fixes:

  - d905a69e boot: boot_serial: Fix upload for swap using offset
    mode
  - 3fc94b38 boot: boot_serial: Fix slot info response on error
  - e2c97da3 boot: bootutil: Fix serial recovery issues
  - 2fcfba13 boot: bootutil: Fix wrong define for single loader
    mode
  - dc571f0b boot: bootutil: Fix single loader trailer size
  - cf92b6f6 boot: boot_serial: Fix wrong slot ID for hook calls
  - c4a7b250 zephyr: cleanup removed
    CONFIG_BOOT_MGMT_CUSTOM_IMG_LIST

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