west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  234c66e66ee39c0f836a57ba805245534be332f2

Brings following Zephyr relevant fixes:

  - 55ae5342 doc: imgtool: Update available options
  - cd96b62f boot: zephyr: Fix comment in banner.c
  - 1357a70f Updates for v2.3.0-rc2 release
  - afcca2c7 boot: zephyr: Fix Espressif build issues
  - d95ca5c7 boot: zephyr: Remove BOOT_MAX_IMG settings
  - 623f0230 boot: zephyr: stm32n657xx: Move overlay to socs
  - 31469b43 boot: zephyr: kconfig: Flip swap move defaults for
    esp32/stm32
  - a2ee0a75 scripts: imgtool: Fix verification with public ed25519
    key file
  - 49bbbabf boot: zephyr: Add watchdog setup/timeout
    configurability options
  - c25d2506 boot: zephyr: Move watchdog code to separate file
  - 9efb9d3f boot: zephyr: kconfig: Move modes to choice Kconfig
  - 92854b7b boot: zephyr: kconfig: Only show signature file when
    needed
  - 716f338a boot: zephyr: Remove weird custom key directory
    handling
  - c52f8af7 boot: zephyr: select `IS_BOOTLOADER`

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