west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  81315483fcbdf1f1524c2b34a1fd4de6c77cd0f4

  - 81315483 Revert "zephyr: arm: Update reading the flash image
    reset vector"
  - 47d826e7 boot: bootutil: swap_offset: Fix maximum application
    size
  - d5d3359e zephyr: Fix trailer size computation for swap-scratch
  - f9e4e529 zephyr: Fix TLV area was included in trailer size when
    rounding up
  - 6cbea0a2 boot: boot_serial: Fix swap using offset
  - efa30399 boot: bootutil: swap_move: Fix maximum application
    size

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