PXE: look for usbethaddr if no ethaddr

Signed-off-by: John Rigby <john.rigby@linaro.org>
diff --git a/common/cmd_pxecfg.c b/common/cmd_pxecfg.c
index ee1b66b..694a212 100644
--- a/common/cmd_pxecfg.c
+++ b/common/cmd_pxecfg.c
@@ -50,6 +50,9 @@
 	ethaddr = from_env("ethaddr");
 
 	if (!ethaddr)
+		ethaddr = from_env("usbethaddr");
+
+	if (!ethaddr)
 		return;
 
 	*outbuf = strdup(ethaddr);