net: lwm2m: Make lwm2m_pull_context reusable

Add a semaphore to control that no collisions occur when multiple
sources want to use the pull_context

Add struct firmware_pull_context *ctx as an argument to the result_cb of
the context. This allows the receiver to do some kind of differentiation
on the source.

Signed-off-by: Michal Ciesielski <michal.m.ciesielski@voiapp.io>