base-files: add function for generating random MAC

This adds a function for generating a valid random MAC address (unset MC
bit / set locally administered bit).

It is necessary for devices which do not have a MAC address programmed
by the manufacturer.

Signed-off-by: David Bauer <mail@david-bauer.net>

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

@@ -174,6 +174,18 @@ macaddr_setbit_la() {
 	macaddr_setbit $1 7
 }
 
+macaddr_unsetbit_mc() {
+	local mac=$1
+
+	printf "%02x:%s" $((0x${mac%%:*} & ~0x01)) ${mac#*:}
+}
+
+macaddr_random() {
+	local randsrc=$(get_mac_binary /dev/urandom 0)
+	
+	echo "$(macaddr_unsetbit_mc "$(macaddr_setbit_la "${randsrc}")")"
+}
+
 macaddr_2bin() {
 	local mac=$1