Implement bootstrap for LwM2M library / sample

[zephyrbot]

Reported by Michael Scott:

Currently, the LwM2M stack does not support DTLS. It's a high priority once the main portion of the library is merged.

(Imported from Jira ZEP-2417)

[zephyrbot]

by Michael Scott:

This ticket includes migrating from net_context_sendto() functions to net_app_send_pkt().

[mike-scott]

DTLS has been implemented for a while. I'm updating this issue name to reflect that only bootstrap is due

[mike-scott]

Done