west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  c8470fb145f8aff92696d05396fb77c3b8068b32

Brings following Zephyr relevant fixes:

  - 2f294d1f zephyr: Use flash_area_flatten in
    bs_custom_storage_erase
  - 2346d69c zephyr: Add support for devices without erase and
    reduced erases
  - 2a2b562b bootutil: Add support for devices without erase and
    reduced erases
  - 1ead4a67 boot: zephyr: kconfig: Remove superflous default n
    lines
  - e5d8640c zephyr: Add missing selection for allowed SHA
    algorithms
  - f523c60d zephyr: Fix ED25519 compilation with mbedTLS
  - 0ba80ffb zephyr: Prevent selecting MBEDTLS_ASN1_PARSE_C when
    not needed
  - ec86244a zephyr: Do not compile ASN1 code when bypassed
  - a01ca4cf bootutil: Fix ASN1 bypass not building
  - 4535ad98 boot: Additional flash_area_open removals
  - c0bb1337 boot: zephyr: boards: Add frdm-mcxn236 configuration

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