mount: Add packet type handlers in mastercomm

Make it possible to delegate handling of particular types of packets
to modules other than mastercomm.

Change-Id: I6670952e257547436055ac5eebb5f02c6062eafb
mount: Sending raw packets in mastercomm

Make it possible to send arbitrary packets to master through mastercomm.

Change-Id: I78089af7cf185bfdffe5cc3d047ef04670ddfe82