riscv: create a loader.bin boot image for Kendryte SoC

Create the loader.bin bootable image file that can be loaded into
Kendryte K210 based boards using the kflash.py tool with the command:

kflash.py/kflash.py -t arch/riscv/boot/loader.bin

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>