base-files: correct global DHCP DUID generation

The old command generates an often incorrect variable length UUID
instead of a fixed 16-octet hex string.

Ref: https://datatracker.ietf.org/doc/html/rfc8415#section-11.5

Signed-off-by: Alice H. <[email protected]>

package/base-files/files/etc/uci-defaults/14_network-generate-clientid

@@ -2,7 +2,7 @@
 
 uci -q batch <<-EOF >/dev/null
 	# DUID-UUID - RFC6355
-	set network.globals.dhcp_default_duid="$(hexdump -vn 16 -e '"0004" 2/2 "%x"' /dev/urandom)"
+	set network.globals.dhcp_default_duid="$(printf '%s%s' '0004' $(cat /proc/sys/kernel/random/uuid | sed -e 's/-//g'))"
 	commit network
 EOF