Implement MAC Flush for BD or Interface from the L2FIB

Implement MAC Flush for BD or Interface from the L2FIB

Allow non-static MACs in the L2FIB which is associated with an
interface or a bridge domain (BD) be flushed. MAC flush are
initiated automatically when an interface is removed from a BD
or when a BD is deleted. MAC flush can also be invoked manually
via the following CLI:
 l2fib mac-flush interface <if-name>
 l2fib mac-flush bridge-domain <bd-id>

Change-Id: Ie33243622834810a765f48ebcd22bdb8e8fc87a4
Signed-off-by: John Lo <loj@cisco.com>