LwM2M client sample: an overlay for OpenThread support

[nduongn]

LWM2M client has a Bluetooth networking sample. We need an overlay configuration sample for Thread networking. The node can connect to a border router.

There is an overlay-ot.conf sample in this issue. But it can not work as I test.

Thanks.

[mike-scott]

@andynnd While I work out how to get this support in mainline Zephyr, you can experiment with it using pre-setup Foundries.io gateway and Zephyr-based samples:
https://docs.foundries.io/latest/tutorial/installation-linux.html
https://docs.foundries.io/latest/tutorial/basic-system.html

For that setup, we have an out-of-the-box demo setup for RPi3, and you will need to follow OpenThread directions for setting up the nRF52840-Dongle as an OpenThread NCP.

[rlubos]

Fixed by #28836.