ext: lib: mgmt: mcumgr: update to latest master

This update to the latest master of mcumgr fixes a memory corruption in
the image management and updates the readme.

Fixes #7924

Origin: mcumgr
License: Apache 2.0
URL: https://github.com/apache/mynewt-mcumgr


commit: a837a731b94927c6198e39744cd6d979be23942a
Purpose: Fix memory corruption
Maintained-by: External

Signed-off-by: Johannes Hutter <johannes@proglove.de>