net: linkaddr: introduce net_linkaddr_set function

The net_linkaddr_storage structure contains an array of bytes used
to store the link address.  This array can be different sizes
depending on the CONFIG options used when building.  To facilitate
consistency and error checking let's introduce a new helper function
to copy the addr and len values to this structure.

Also move all uses of memcpy related to net_link_storage structures to
the new helper function.

Change-Id: Ic547d86b07e62e5ac3bc330d4eaeb4508a143200
Signed-off-by: Michael Scott <michael.scott@linaro.org>