[coreboot] GPL license headers

[Patrick Georgi]

One issue brought up the last time we had this discussion was that
lawyers prefer to have their tools (eg. license scanners) continue to
work, which is why we shouldn't reduce the length too much (we had a
number of proposals with just two lines back then). Referring to an
external file is alright, as long as you assume that the code never
leaves the context of the repository.


Patrick
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