This commit consists of..
(1) Part of Wojciech's changes to ethernet-packet.yang.
(2) EthernetDecoder refactoring.
(3) ArpDecoder fix to account for the offset field being in bytes.

Change-Id: I452665e5593e7a5f380ccdf96154633f6a8eea14
Signed-off-by: Alex Fan <railor33@gmail.com>