net: lwm2m: Report boostrap complete after final response is sent
net: lwm2m: Report boostrap complete after final response is sent

So far, `LWM2M_RD_CLIENT_EVENT_BOOTSTRAP_TRANSFER_COMPLETE` event was
reported before the final ACK for the Bootstrap Finish was sent from the
client side. This could cause delays in the ACK sending, in case the
application wanted for instance to store the received data in flash.

Fix this, by reporting the
`LWM2M_RD_CLIENT_EVENT_BOOTSTRAP_TRANSFER_COMPLETE` event on the next
state tansition (before the actual registration starts).

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

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