net: lwm2m: add firmware push support

1. Add handling block1 option in handle_request(). The basic idea is
   to declare structure block_context at compiled time and use "token"
   as a key to pick up the on-going block cotext. It should be able to
   support multiple blockwise transfer concurrently
2. Use write callback implemented in lwm2m_obj_firmware to deal w/ the
   update state transition and than call the callback registered by the
   application
3. move default_block_size to lwm2m_engine.c to share between
   lwm2m_engine and lwm2m_obj_firmware_pull

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>
[michael.scott@linaro.org: rebased on LwM2M net_app changes.]
Signed-off-by: Michael Scott <michael.scott@linaro.org>