libusbgx: Allow to pass usbg_init() without access to /sys/class/udc

libusbgx: Allow to pass usbg_init() without access to /sys/class/udc

We may would like to use this library on machine where
is no udc support (no /sys/class/udc directory) or
where we don't have sufficient rights to read udc
directory.

This commit adds support for such situation. If we are
running on such machine we will be able to pass usbg_init()
function and manage usb gadgets but we won't be able to bind
gadget to udc as we were unable to read their names.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Reviewed-by: Pawel Szewczyk <p.szewczyk@samsung.com>