usb-gadget: introduce configurable function set

usb-gadget: introduce configurable function set

Previously, only a few predefine set of usb-gadget function can be
utilized for setting usb-gadget, e.g., SET_USB_SDB_DIAG means it only
activates usb-gadget functions of sdb/diag, and you are not allowed
to activates any other usb-gadget combination than those predefined
set.

For a better intuitive useability, remove those predefined set. And
you are now allowed to combine any kind of usb-gadget functions you
want to use, making up a bitmap, which can be used for activating
all those usb-gadget functions through vconf(db/usb/sel_mode).

Usb-gadget functions are defined in capi-system-device.
 - USB_GADGET_FUNC_NONE  = 0x00
 - USB_GADGET_FUNC_SDB   = 0x01
 - USB_GADGET_FUNC_MTP   = 0x02
 - USB_GADGET_FUNC_RNDIS = 0x04
 - USB_GADGET_FUNC_ACM   = 0x08
 - USB_GADGET_FUNC_DIAG  = 0x10
 - USB_GADGET_FUNC_RMNET = 0x20

Change-Id: I44c2bb2c751129613a3470569edf8babbe710845
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>