mgmt/mcumgr/lib: Remove zephyr_ and _impl_ from function names

The commit removes zephyr_ and _impl_ from function names in image
management group, and renames img_mgmt_impl.* source files to
img_mgmt_priv and merges img_mgmt_priv.h headers.
The zephyr_ and _impl_ have been removed because they no longer make
sense, as the mcugr is internal part of Zephyr, and removal makes
function names shorter.

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