net: lwm2m: add LWM2M_FIRMWARE_UPDATE_PULL_LOCAL_PORT setting

This allows a user to customize the port used for downloading
firmware via the pull method of the LwM2M client.  It's default
value of 0 will select a random port during initialization.

NOTE: If set, this value should not be the same port as the
LWM2M_LOCAL_PORT setting.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>