mesh: Handle messages sent to a fixed group address

mesh: Handle messages sent to a fixed group address

This handles the case when an inbound message is addressed to
a fixed group, i.e., all-proxies, all-friends, all-relays and
all-nodes. The message is delivered to a primary element only,
and, with the exception of all-nodes case, if the corresponding
feature is enabled on the node.

Change-Id: Ibdc2df9d180b902ce4089d2fd16f2f5d0dfbabb0
Signed-off-by: Anupam Roy <anupam.r@samsung.com>