west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  bf5321bb6906e9e75b4c85b7c98b71c06ae52e90

Brings following Zephyr relevant fixes:

  - bf5321bb boot: zephyr: Remove reference to deprecated BT_CTLR
    option
  - 800868a9 bootutil: swap_scratch: Fix scratch scrambling
  - 16cfdaf3 boot: zephyr: Fix including asn1 when ed25519 is used

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>