[coreboot] Directly boot the image of Linux for RISC-V

[王翔]

Hello Dear:

I added some code to support linux binary directly as a payload to boot for RISC-V.

modify:
    1. opensbi is an open source sbi implementation(https://github.com/riscv/opensbi)
    2. Linux binary is used as the payload of opensbi
    3. The elf generated by opensbi compilation is used as the coreboot payload.

Code reference:
    https://github.com/hardenedlinux/coreboot-HiFiveUnleashed/tree/r1

Whether to consider adding opensbi to 3rdparty?
Do you have any suggestions?

XiangWang

------------------

王翔

安全研究员

广州市腾御安信息科技有限公司

广州市天河区珠江新城华穗路406号保利克洛维二期中景A座1020-1024