image: Add a common compression type detection function.

Currently, there is no method that can detect compression types
given a file. This is very useful where a compressed kernel image
is loaded directly to the memory.

Inspect initial few bytes to figure out compression type of the
image. It will be used in booti method for now but can be reused
any other function in future as well.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Tom Rini <trini@konsulko.com>