base-files: rename hostname with EUI of mac address

If a label MAC address is provided for device, system
will rename the hostname with OpenWrt_{eui mac address}.
This helps to distinguish between different devices.

Since it's no good idea to nest json_* functions, this code does
not use get_mac_label directly, but only get_mac_label_dt as
external resource.

Signed-off-by: Rosy Song <rosysong@rosinson.com>
[merged with commit introducing macaddr_geteui, rebased on updated
label MAC address storage, extended commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

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

@@ -144,6 +144,13 @@ macaddr_add() {
 	echo $oui:$nic
 }
 
+macaddr_geteui() {
+	local mac=$1
+	local sep=$2
+
+	echo ${mac:9:2}$sep${mac:12:2}$sep${mac:15:2}
+}
+
 macaddr_setbit_la() {
 	local mac=$1