net: lwm2m: setup lwm2m ctx for firmware download

Previously, firmware support wasn't initializing the retransmit work
or the extra network packet pools.  Let's fix that.

NOTE: While this fixes the setup of retransmit work, the actual
attempts to re-send packets which are pending is failing.  Needs
another follow-up fix.

Signed-off-by: Michael Scott <michael.scott@linaro.org>