base-files: reduce IPv6 ULA prefix generation to a single call

Reduce calls and pipes and read from urandom once directly with hexdump
for the necessary 5 bytes of random data to build the 48 bit ULA Prefix.

Fewer calls and forks; finish quicker; less memory used.

Tested on: 23.05.3

Signed-off-by: Paul Donald <[email protected]>

package/base-files/files/etc/uci-defaults/12_network-generate-ula

@@ -1,11 +1,7 @@
 [ "$(uci -q get network.globals.ula_prefix)" != "auto" ] && exit 0
 
-r1=$(dd if=/dev/urandom bs=1 count=1 |hexdump -e '1/1 "%02x"')
-r2=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')
-r3=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')
-
 uci -q batch <<-EOF >/dev/null
-	set network.globals.ula_prefix=fd$r1:$r2:$r3::/48
+	set network.globals.ula_prefix="$(hexdump -vn 5 -e '"fd" 1/1 "%02x:" 2/2 "%x:"' /dev/urandom):/48"
 	commit network
 EOF