PXE: look for usbethaddr if no ethaddr

Signed-off-by: John Rigby <john.rigby@linaro.org>
diff --git a/common/cmd_pxe.c b/common/cmd_pxe.c
index ee75db9..ed09d21 100644
--- a/common/cmd_pxe.c
+++ b/common/cmd_pxe.c
@@ -61,6 +61,9 @@
 	ethaddr = from_env("ethaddr");
 
 	if (!ethaddr)
+		ethaddr = from_env("usbethaddr");
+
+	if (!ethaddr)
 		return -ENOENT;
 
 	ethaddr_len = strlen(ethaddr);