base-files: add function to retrieve label MAC address

To refer to the MAC address on a device's label, one can
specify the alias label-mac-device in the DTS which should
point to the bearer of the corresponding MAC address.

With the function get_mac_label, the user can retrieve then
retrieve this address and use it as a value that uniquely
identifies his device.

This is severely helpful for several downstream functionalities,
e.g. define MAC addresses of custom netifs or change the SSID to
be easily recognizable.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/lib/functions/system.sh

@@ -12,6 +12,16 @@ get_mac_binary() {
 	hexdump -v -n 6 -s $offset -e '5/1 "%02x:" 1/1 "%02x"' $path 2>/dev/null
 }
 
+get_mac_label() {
+	local basepath="/proc/device-tree"
+	local macdevice="$(cat "$basepath/aliases/label-mac-device" 2>/dev/null)"
+	local macaddr
+
+	[ -n "$macdevice" ] && macaddr=$(get_mac_binary "$basepath/$macdevice/mac-address" 0 2>/dev/null)
+	[ -n "$macaddr" ] || macaddr=$(get_mac_binary "$basepath/$macdevice/local-mac-address" 0 2>/dev/null)
+	echo $macaddr
+}
+
 find_mtd_chardev() {
 	local INDEX=$(find_mtd_index "$1")
 	local PREFIX=/dev/mtd