net: don't override ethernet address environment

If the ethernet address environment is set with a valid
ethernet address prevent overriding it as it is most likely
set by the user and he/she doesn't want board code to
automatically override it whatsoever.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

net/eth_common.c

@@ -34,6 +34,9 @@ int eth_setenv_enetaddr(const char *name, const uchar *enetaddr)
 {
 	char buf[ARP_HLEN_ASCII + 1];
 
+	if (eth_getenv_enetaddr(name, (uchar *)buf))
+		return -EEXIST;
+
 	sprintf(buf, "%pM", enetaddr);
 
 	return setenv(name, buf);