riscv: boot: add zstd support

Support build the zstd compressed Image.zst. Similar as other
compressed formats, the Image.zst is not self-decompressing and
the bootloader still needs to handle decompression before
launching the kernel image.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20221123150257.3108-1-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>