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

This patch introduce version which add two new features:
 - ability to resume partial upload
 - option to not compile taskstat and echo commands

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


Commit: 6251689367fcfe92898b90978b877a242b6e4b24
Purpose: New features
Maintained-by: External

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>