Move mcumgr code back to core

At time mcumgr was moved to separate repo [1] and were enhanced with adaptation layer to help useage by other embedded projects.
After some time it turns out that projects prefer to have own fork instead, so this is no longer needed.

- move code back to core
- remove non-mynewt code
- remove adaptation layer
- archive [1]

[1] https://github.com/apache/mynewt-mcumgr