net: lwm2m: enable monitoring of fw update state/result

The intention behind this patch is to know the current state/result
of a firmware update process in the application code. It makes it
possible to use pre/post_write_callbacks to get the proper value
of state (5/0/3) and result (5/0/5) resource.

Signed-off-by: Andreas Chmielewski <andreas.chmielewski@grandcentrix.net>