[U-Boot] x86: Slim Bootloader(https://github.com/slimbootloader/slimbootloader) support

Hi Simon and Bin,

I am a firmware engineer from Intel Corporation. We have developed new boot solution - Slim Bootloader(https://github.com/slimbootloader/slimbootloader) which currently supports qemu, Apollolake, Whiskeylake and Coffeelake platforms with FSP2.x. The Slim Bootloader is also designed with multi-stage approach and payload concept. We have a proof of concept with u-boot as a payload of Slim Bootloader and would like to upstream the changes into u-boot repository. I have a single patch for it and I am going to send it for code review. Do you have any comment or question before proceeding the code review? Please feel free to add any comment. Thanks.

Best Regards, Aiden