Can not extract initrd.img by using extract_initrd

```
$./bin/extract_initrd initrd.img
Extracting initrd ramdisk...

gzip: stdin: not in gzip format
```

```
$ file boot/initrd.img 
boot/initrd.img: data
```

And it's not xz compressed, not ext2/ext4 image

phicomm N1 S905D soc.