mv_feroceon: default eth and pon mac address envs to sysvar equivalents.

The eth and pon mac addresses are passed to Linux via a Marvell ARM tag. The
value of those tags comes from the eth and pon mac address environment
variables.

This change seeds those envs with the corresponding sysvars ETH_MAC_ADDR and
PON_MAC_ADDR (if they exist).

Change-Id: I2e4b4a158f37d127ff51f0355c161edcab31612a
diff --git a/board/mv_feroceon/mv_kw2/mv_main.c b/board/mv_feroceon/mv_kw2/mv_main.c
index 8c762ec..a434aa2 100644
--- a/board/mv_feroceon/mv_kw2/mv_main.c
+++ b/board/mv_feroceon/mv_kw2/mv_main.c
@@ -818,6 +818,10 @@
 	sprintf(ethaddr_1,"00:50:43:%02x:%02x:%02x",xl,xi,xj);
 	sprintf(pon_addr,"00:50:43:%02x:%02x:%02x",xj,xk,xl);
 
+	/* sf_getvar overwrites the buffer only when the sysvar exists */
+	sf_getvar("ETH_MAC_ADDR", ethaddr_0, sizeof(ethaddr_0));
+	sf_getvar("PON_MAC_ADDR", pon_addr, sizeof(pon_addr));
+
 	/* MAC addresses */
 	env = getenv("ethaddr");
 	if(!env)