usbgadget: autostart: add DFU support ...

Use global variable dfu_function to autostart DFU. As similar code
is used to start multifunction gadget using command, move common
code to common/usbgadget.c and consolidate it.
It turned out that '-s' option of usbgadget command does nothing,
so remove its help text and make it function as '-a'.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>