cmd/mkinitfs: Exclude initramfs files from initramfs-extra (MR 34)

When testing on pmOS with qemu/x86_64, this results in some nice
reduction in size of the initramfs-extra (gzip'd, default compression):

Before:
        /mkinitfs # ls -la /boot/initramfs-extra
        -rw-r--r--    1 root     root       3544429 Mar 19 23:06 /boot/initramfs-extra

After:
        /mkinitfs # ls -la /boot/initramfs-extra
        -rw-r--r--    1 root     root       2234020 Mar 19 23:08 /boot/initramfs-extra

Fixes #23

cmd/mkinitfs/main.go

@@ -141,7 +141,7 @@ func main() {
 		modules.New([]string{}, "/etc/mkinitfs/modules-extra"),
 		osksdl.New(devinfo.MesaDriver),
 	})
-	initramfsExtraAr.AddItems(initfsExtra)
+	initramfsExtraAr.AddItemsExclude(initfsExtra, initfs)
 	if err := initramfsExtraAr.Write(filepath.Join(workDir, "initramfs-extra"), os.FileMode(0644)); err != nil {
 		log.Println(err)
 		log.Println("failed to generate: ", "initramfs-extra")