boot: Support compressed booti images in bootm

boot: Support compressed booti images in bootm

A compressed booti image relies on the compression-format's header at
the start to indicate which compression algorithm is used.

We don't support this elsewhere in U-Boot, so assume that a compressed
file is always a booti file. Once it is compressed, a check is made to
make sure that it actually is.

Simplify the implementation by adding a new function which returns the
booti image-type if compression is detected.

Signed-off-by: Simon Glass <sjg@chromium.org>