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I had a discussion with U-Boot Creator and Core Developer Wolfgang Denk.
He made some patches, they were accepted and the result is this header:
# (C) Copyright 2000-2013
# Wolfgang Denk, DENX Software Engineering, wd@....
# SPDX-License-Identifier: GPL-2.0+

and a Licenses folder with a README, Exception and the Licenses:
http://git.denx.de/?p=u-boot.git;a=tree;f=Licenses;hb=HEAD

see also mailing list details here:
http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/165416
or this web site: http://www.denx.de/wiki/U-Boot/Licensing

It is not applied to all files of U-Boot. However, more than 5760 files
already use this new header.

a very resourceful way to simplify the license topic and an important step
towards adoption of SPDX
usage within projects and SPDX files provided with OSS releases.

all the best!
-roger

PS: similar discussions at Apache:
https://issues.apache.org/jira/browse/LEGAL-114