[riscv64] Change the kernel image format from ELF to flat Image.

(Closes: #928451)

The riscv64 architecture is changing its standard kernel image
format from ELF to a flat kernel image with a PE/COFF-compatible
header (similar to arm64) to make EFI stub support possible.
Ship arch/riscv/boot/Image instead of an ELF vmlinux in
accordance with this change.