At present U-Boot has a header file called lz4.h for its own use. If the
host has its own lz4 header file installed (e.g. from the 'liblz4-dev'
package) then host builds will use that instead.

Move the U-Boot file into its own directory, as is done with various
other headers with the same problem.

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

Add a header file to house the lz4 compression function. Add a comment
while we are here, since it not even clear from the name what the function
actuall does.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>