west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  6d3b3d2c38ab20c242e5b9abb04d050086383eb2

Brings following Zephyr relevant fixes:

  - 6d3b3d2c Version bump for v2.4.0
  - f58e96a0 bootutil: zephyr: link to `mbedTLS` only when
    `CONFIG_MBEDTLS_BUILTIN`
  - 16996904 bootutil: Fix format string in swap_run function log

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