USB: g_multi: Multifunction Composite Gadget added

The Multifunction Composite Gadget has two configurations
consisting of Ethernet (RNDIS in first and CDC Ethernet in
second configuration), CDC Serial and File-backed Storage
functions.

When connected to a Windows host, the first configuration
is chosen thus gadget provides RNDIS Ethernet, serial and
mass storage whereas when connected to Linux host, second
configuration is chosen thus providing CDC Ethernet,
serial and mass storage.

Which configurations are built can be configured via
KConfig options.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>