FunctionFS: initial implementation

This is the second version of a patch which demonstrates the possibility
of using adbd (Android Debug Bridge daemon) with a generic FunctionFS gadget
instead of a custom adb usb gadget in the Linux kernel. It contains changes
introduced after Benoit's review - thank you Benoit.

The patch adds a new usb access layer to adbd using FunctionFS. The former
usb access method is still available. The method is chosen at runtime
depending if /dev/usb-ffs/adb/ep0 or /dev/android_adb is accessible.

How to use on the target device:

$ insmod g_ffs.ko idVendor=<vendor ID> iSerialNumber=<some string>
$ mount -t functionfs adb /dev/usb-ffs/adb -o uid=2000,gid=2000
$ ./adbd

This patch requires a patch to bionic which adds <linux/usb_functionfs.h>
which is an exact copy of the relevant file in the linux kernel.

Change-Id: I4b42eb267ffa50fca7a5fba46f388a2f083e8b2d
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[benoit@android.com: detect at runtime if functionfs is mounted
or fallback using f_adb]
Signed-off-by: Benoit Goby <benoit@android.com>