usb: gadget: Add support for more than one configuration in android gadget

This feature allows defining more than one configuration to an android
usb gadget. This is important if we want our device to be compatible
with different kinds of host PCs. For example Windows host or Linux host.
Each host can select the configuration it supports from the several
configurations that our device exposes.

In order to define more then one configuration in a composition,
use the functions sysfs attribute of the android gadget, by using
a ':' character to separate between the configurations.
For example: echo "rndis,adb:ecm" > android0/functions

Using this feature there is one restriction, each usb configuration should
include different set of function drivers so a function driver cannot
appear in more then one usb configuration at the same time.

Change-Id: Id8ca2ee3dcc3257bc56cc7b7b8a9ecbbd6316d45
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>