manifest: Update MCUboot revision

Updates MCUboot to commit 6902abb from upstream which includes:

6902abb zephyr: Create common boot serial enter function
35941fe boot: zephyr: Add pin reset serial recovery entrance method
fd79db3 zephyr: boot: serial_recovery: Add no application entrace
        method
b3e3ce3 boot: zephyr: serial_recovery: Add boot mode enter ability
e5c57dd boot_serial: Only have build number if non-zero
8a3b32c bootutil: Refactor signature verification
4e07d8f docs: add release note snippet for dumpinfo
ca56135 imgtool: Add 'dumpinfo' command
c050573 docs: Add release note for p224 removal
75c7c31 sim: Remove P224 curve references
3d92a6c imgtool: Remove P224 curve references
206b914 bootutil: Remove P224 curve
a97f009 Update readme for next dev version
23d2883 Update to version 1.10.0
cdf9de0 doc/readme-zephyr: document the serial recovery
4e9d86a doc: Added serial recovery documentation
827118f boot: serial_recovery: Add image hash support
f5e7753 boot_serial: support fragmentation for outgoing SMP packets
569b1d6 Update to version 1.10.0-rc1
d2dfa1b docs: Create release notes for 1.10.0 release
4337fee docs: Describe the release note snippet requirement
d4184eb docs: Setup template for release note process
1090d8f zephyr: Check zephyr,uart-mcumgr as candidate for serial
        recovery

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