subsys/mgmt: Enabling mynewt-core version of tinycbor

This PR provides changes that are required after replacing Tinycbor
with copy of source code from mynewt-core.
The Tinycbor has been replaced with mynewt-core version to reduce
maintenance effort; by replacing it the Zephy specific changes have been
reduced to small patch over mynewt codebase.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>