net: lwm2m: add state machine for firmware pull update

1. Parse firmware pull URI
2. Add lwm2m_firmware_get/set_update_cb() for application to register
   callback. This is because we want to check the update_state before
   we pass to the application
3. Add lwm2m_firmware_get/set_update_result() and
   lwm2m_firmware_get/set_update_stat() to manage the state transition
   as well as the sanity check

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>
[michael.scott@linaro.org: rebased on net_app framework and
lwm2m_message refactoring.]
Signed-off-by: Michael Scott <michael.scott@linaro.org>