Add support for LZ4 decompression algorithm

This patch adds support for LZ4-compressed FIT image contents. This
algorithm has a slightly worse compression ration than LZO while being
nearly twice as fast to decompress. When loading images from a fast
storage medium this usually results in a boot time win.

Sandbox-tested only since I don't have a U-Boot development system set
up right now. The code was imported unchanged from coreboot where it's
proven to work, though. I'm mostly interested in getting this recognized
by mkImage for use in a downstream project.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>