manifest: Mcuboot update

Mcuboot was updated to the version synchronized with
the upstream 5b7ed6a which introduces:
- Added LED support for signaling enter to the serial recovery mode
- Added boot delay/debonuce of serial detect pin.
- Fixed compilation warnings with ZEPHYR_LOG_MODE_MINIMAL
- Added optional check which prevents attempting to boot image that
has been build for different ROM address than a slot it currently
resides in. Check is enabled if image was signed with
IMAGE_F_ROM_FIXED flag.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/west.yml b/west.yml
index d99fdcd..565ef6e 100644
--- a/west.yml
+++ b/west.yml
@@ -91,7 +91,7 @@
       revision: 24d84ecff195fb15c889d9046e44e4804d626c67
       path: modules/crypto/mbedtls
     - name: mcuboot
-      revision: 3fc59410b633a6d83bbb534e43aac43160f9bd32
+      revision: 846b104e67c46f9428fd020ba8dccf94f786d94d
       path: bootloader/mcuboot
     - name: mcumgr
       revision: c52ecb771a9b2bdabfc70bee094555f11edbb539