[coreboot] Intel EDK2 header files

[Jonathan Zhang (Infra)]

Hi coreboot colleagues,

Intel EDK2 header files are needed to build coreboot that depends on FSP. So far the practice is
to keep such header files under src/vendorcode/intel/edk2 directory, and each EDK2 version takes
a different directory (we have uefi_2.4, UDK2015, UDK2017).

Since CPX-SP FSP is based on latest EDK2 code base, I would like to add EDK2 quarterly release
(edk2-stable202005) header files to coreboot code base, and created [CB: 42239] accordingly.

Idwer commented that an alternative is to keep EDK2 header files as a submodule, and this should
be discussed in coreboot community.

Thanks,
Jonathan