image: android: Automatically detect more compression types

At the moment android_image_get_kcomp() can automatically detect
LZ4 compressed kernels and the compression specified in uImages.
However, especially on ARM64 Linux is often compressed with GZIP.
Attempting to boot an Android image with a GZIP compressed kernel
image currently results in a very strange crash, e.g.

  Starting kernel ...
  "Synchronous Abort" handler, esr 0x02000000
   ...
  Code: 5555d555 55555d55 555f5555 5d555d55 (00088b1f)

Note the 1f8b, which are the "magic" bytes for GZIP images.

U-Boot already has the image_decomp_type() function that checks for
the magic bytes of bzip2, gzip, lzma and lzo. It's easy to make use
of it here to increase the chance that we do the right thing and the
user does not become confused with strange crashes.

This allows booting Android boot images that contain GZIP-compressed
kernel images.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>