util/lint & LICENSES: Add PDDC as a "license" for coreboot

The Creative Commons Public Domain Dedication and Certification is not
a license in the common sense in that it's stating that the associated
file is already in the public domain (having no copyright), and is not
actually putting it in the public domain like the CC-C0 license does.

The use for this in coreboot is for unlicensable files - either blank
files or files with no creative content.  This allows these files to
have the SPDX identifier to identify them as having no known copyright
for open source license compliance.

If CC-PDM-1.0 is ever included in the list of SPDX licenses, that would
probably fit better, but because the public domain mark isn't actually
a license, and because "public domain" isn't well defined, CC-PDM was
rejected as a SPDX identifier.

For further information:
https://web.archive.org/web/20201018194411/https://github.com/spdx/license-list-XML/issues/988

Change-Id: Ibb300ecd066cde2a016195c2beca76a460c588e3
Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66496
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

LICENSES/CC-PDDC.txt

@@ -0,0 +1,27 @@
+
+The person or persons who have associated work with this document (the
+"Dedicator" or "Certifier") hereby either (a) certifies that, to the best of
+his knowledge, the work of authorship identified is in the public domain of
+the country from which the work is published, or (b) hereby dedicates whatever
+copyright the dedicators holds in the work of authorship identified below (the
+"Work") to the public domain. A certifier, moreover, dedicates any copyright
+interest he may have in the associated work, and for these purposes, is
+described as a "dedicator" below.
+
+A certifier has taken reasonable steps to verify the copyright status of this
+work. Certifier recognizes that his good faith efforts may not shield him from
+liability if in fact the work certified is not in the public domain.
+
+Dedicator makes this dedication for the benefit of the public at large and to
+the detriment of the Dedicator's heirs and successors. Dedicator intends this
+dedication to be an overt act of relinquishment in perpetuity of all present
+and future rights under copyright law, whether vested or contingent, in the
+Work. Dedicator understands that such relinquishment of all rights includes the
+relinquishment of all rights to enforce (by lawsuit or otherwise) those
+copyrights in the Work.
+
+Dedicator recognizes that, once placed in the public domain, the Work may be
+freely reproduced, distributed, transmitted, used, modified, built upon, or
+otherwise exploited by anyone for any purpose, commercial or non-commercial,
+and in any way, including by methods that have not yet been invented or
+conceived.

util/lint/lint-000-license-headers

@@ -116,6 +116,12 @@ check_for_license 'SPDX-License-Identifier: X11'
 # differentiate between this license with or without advertising.
 check_for_license 'SPDX-License-Identifier: BSD-4-Clause-UC'
 
+# This is the Creative Commons Public Domain Dedication and Certification
+# for files that are already in the public domain.  This includes files
+# that cannot be licensed such as blank files or files with no "creative"
+# content.
+check_for_license 'SPDX-License-Identifier: CC-PDDC'
+
 for file in $headerlist; do
 	# Verify the file exists, and has content that requires a header
 	# This assumes that a file that has 4 lines or fewer is not notable