[coreboot] UEFI Payload update

[You, Benjamin]

Hi,

A staging project "UEFIPayload" has been started to upgrade the Coreboot*Pkgs in EDK II repository.

The new features are:
- Supporting Slim Bootloader (https://github.com/slimbootloader) in addition to Coreboot
- Source file configuration using .ini format
- User extension using simple "C" libs
- Platform supporting lib

The link of this project is: https://github.com/tianocore/edk2-staging/tree/UEFIPayload

Thanks,

- ben