mgmt: smp: MCUmgr Client support

MCUmgr client basic implementation for support Image and OS grpup
commands.

Image Group:
* Image state read/write
* Image Upload secondary slot
* Image Erase secondary slot

OS group:
* Reset
* Echo service, disabled by default

Opeartion's are blocked call and cant't call inside worker queue.
IMG and OS need to be SMP client object for transport.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>