RISC-V: Add Linux load logic

RISC-V: Add Linux load logic

We currently only support to run grub on RISC-V as UEFI payload. Ideally,
we also only want to support running Linux underneath as UEFI payload.

Prepare that with some Linux boot stub code. Once the arm64 target is
generalized, we can hook into that one and gain boot functionality.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>