Makefile: Allow cross-compilation for RISC-V Allow setting FIRMWARE_ARCH=riscv to build vboot for the RISC-V architecture. Note that building for 32-bit RISC-V requires additional CFLAGS. Change-Id: I5c72900494f465fb625f315f550849f5b69f3cca Signed-off-by: Carlos López <[email protected]> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6091368 Tested-by: Julius Werner <[email protected]> Commit-Queue: Julius Werner <[email protected]> Reviewed-by: Yu-Ping Wu <[email protected]> Reviewed-by: Julius Werner <[email protected]>