net: lwm2m: split out lwm2m context initialization

Create an internal function lwm2m_engine_context_init() which sets
the extra packet pools and initializes retransmit work internal to
the LwM2M engine.

This function will be used by firmware pull support which establishes
a new LwM2M context for downloading firmware.

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