kirkwood: cleanup nsa310b mac address extraction

switch nsa310b mac address reading to mtd_get_mac_ascii helper as
it seems the fw_env.config file is created way later than when
network is set up, when I tested I still had that file included
in the image through /files folder.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>

target/linux/kirkwood/base-files/etc/board.d/02_network

@@ -32,7 +32,7 @@ case "$board" in
 	;;
 "nsa310b")
 	ucidef_set_interface_lan "eth0" "dhcp"
-	ucidef_set_interface_macaddr "lan" $( fw_printenv ethaddr | awk -F"=" '{print $2}' )
+	ucidef_set_interface_macaddr "lan" $( mtd_get_mac_ascii uboot_env ethaddr )
 	;;
 *)
 	ucidef_set_interface_lan "eth0"