riscv/purgatory: Omit use of bin2c

The .incbin assembler directive is much faster than bin2c + $(CC).

Do similar refactoring as in commit 4c0f032d4963 ("s390/purgatory:
Omit use of bin2c").

Please note the .quad directive matches to size_t in C (both 8 byte)
because the purgatory is compiled only for the 64-bit kernel.
(KEXEC_FILE depends on 64BIT).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220625223438.835408-2-masahiroy@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

RISC-V: Add purgatory

This patch adds purgatory, the name and concept have been taken
from kexec-tools. Purgatory runs between two kernels, and do
verify sha256 hash to ensure the kernel to jump to is fine and
has not been corrupted after loading. Makefile is modified based
on x86 platform.

Signed-off-by: Li Zhengyu <lizhengyu3@huawei.com>
Link: https://lore.kernel.org/r/20220408100914.150110-6-lizhengyu3@huawei.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>