RISC-V: Add multiple compression image format.

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>