odroid: Update README with USB host usage

Add information wrt using the USB host interface for loading kernel over
ethernet and/or usb mass storage.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>