Currently, there is only support for .gz compression type
for generating kernel Image.

Add support for other compression methods(lzma, lz4, lzo, bzip2)
that helps in generating a even smaller kernel image. Image.gz
will still be the default compressed image.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>