west.yaml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  a6aef32619455cb1697c42d66db489c1608f8410

Brings following Zephyr relevant fixes:
 - d6a7741 zephyr: Add VERSION file
 - fac2cab boot_serial: Add image state set/get
 - c393b54 boot: boot_serial: fix usage of zcbor_new_encode_state
           API
 - 9380135 ci: zephyr: Update Zephyr image and SDK version
 - 9ced459 boot: zephyr: fix s/junping/jumping/ typo
 - 1dbe0cf boot: zephyr: Use mcuboot-led0 in MCUBOOT_INDICATION_LED
           help section
 - 256bc37 bootutil: Fixing memset not beeing called
 - eb7658e zephyr: fix link to Zephyr application docs
 - cb07e88 boot_serial: Replace cbor auto-generated code with zcbor
           functions
 - db6ba46 boot_serial: Unify zcbor include paths

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