west.yaml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  11ecbf639d826c084973beed709a63d51d9b684e

Brings following Zephyr relevant changes and fixes:
 - 11ecbf63 zephyr: use cmsis_core.h header
 - da65db00 zephyr: Provide slot definitions for three images
 - 2c61caf6 bootutil: Move flash_area_id_to_image under ifdef
 - 904d0c46 bootutil: Add DirectXIP version of boot_set_next
 - 2a874b6e zephyr: Do not build in debug mode
 - 258a6c7d bootutil: Fix support for more than 2 flash areas

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>