RISCV: image: Add booti support

This patch adds booti support for RISC-V Linux kernel. The existing
bootm method will also continue to work as it is.

It depends on the following kernel patch which adds the header to the
flat Image. Gzip compressed Image (Image.gz) support is not enabled with
this patch.

https://patchwork.kernel.org/patch/10925543/



Tested on HiFive Unleashed and QEMU.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Karsten Merker <merker@debian.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>