riscv: make image compression configurable

riscv: make image compression configurable

Previously the build process would always set KBUILD_IMAGE to the
uncompressed Image file (unless XIP_KERNEL or EFI_ZBOOT was enabled) and
unconditionally compress it into Image.gz. However there are already
build targets for Image.bz2, Image.lz4, Image.lzma, Image.lzo and
Image.zstd, so let's make use of those, make the compression method
configurable and set KBUILD_IMAGE accordingly so that targets like
'make install' and 'make bindeb-pkg' will use the chosen image.

Tested-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240504193446.196886-2-emil.renner.berthing@canonical.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>