riscv64: add new port on 5.10

riscv64: add new port on 5.10

RISC-V is a new CPU architecture aimed to be fully free and open. This
target will add support for it, based on 4.19.

Supports running on:
 - HiFive Unleashed (which is the most known devboard for this arch)
 - QEMU (support is available in mainline qemu)
Further devboards are expected given the interest in this new arch.

An SD-card image is generated, where the partitions are required to have
specific type codes. Compared to earlier branches for this target, BBL
support is removed, giving way for OpenSBI.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>