west.yaml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  76d19b3b8885ea7ae25a6f4f5d8501f7ec646447

Brings following Zephyr relevant fixes:
 - 76d19b3 boot: bootutil: Fix missing packed attributes, add hash
           size define
 - 018b770 imgtool: Fix getpriv error return with private key
 - 9fad4c1 boot: boot_serial: Fix wrong cbor type for confirm

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