spl: support for booting via usbeth

In case of usbeth booting just call net_load_image("usb_ether").
This patch also adds CONFIG_SPL_USBETH_SUPPORT and
CONFIG_SPL_MUSB_NEW_SUPPORT config options to enable linking of SPL
against USB gagdet support and new MUSB driver resp.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>