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>
diff --git a/common/spl/spl.c b/common/spl/spl.c
index ff9ba7b..6a5a136 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -221,6 +221,11 @@
 #endif
 		break;
 #endif
+#ifdef CONFIG_SPL_USBETH_SUPPORT
+	case BOOT_DEVICE_USBETH:
+		spl_net_load_image("usb_ether");
+		break;
+#endif
 	default:
 		debug("SPL: Un-supported Boot Device\n");
 		hang();