[coreboot] [PATCH] tint: Add libpayload/tint patch + README

[Uwe Hermann]

See patch.

I've added the patch and a small README (which should be extended later)
into payloads/external/tint, in order to separate external payloads
which we simply port to libpayload from "native" coreboot payloads such
as coreinfo.

Uwe.