net: lwm2m: Introduce ENGINE_IDLE state
net: lwm2m: Introduce ENGINE_IDLE state

So far, the LwM2M state machine started in the `ENGINE_INIT` state,
which made it exectue the registration/bootstrap registration even when
`lwm2m_rd_client_start()` was not called. With a new `ENGINE_IDLE`
state, the state machine can wait for the application to actually start
the client before proceeding. It also makes sense to stay in the
ENGINE_IDLE state after successfull deregistration, until the
application restarts the client.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

nashif/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr