[M] Change in coreboot[master]: mainboard: Add SPDX license headers to Makefiles

[Michael Niewöhner (Code Review)]

Attention is currently required from: Felix Singer, Jason Nien, Jeremy Soller, Angel Pons, Martin Roth, Tim Crawford, Nico Huber, Tarun Tuli, Martin L Roth, Damien Zammit, Subrata Banik, Maxim Polyakov, Kapil Porwal, Alexander Couzens, Werner Zeh, Evgeny Zinoviev.

Michael Niewöhner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/68982 )

Change subject: mainboard: Add SPDX license headers to Makefiles
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Patch Set 2:

(2 comments)

File src/mainboard/clevo/cml-u/variants/l140cu/Makefile.inc:

https://review.coreboot.org/c/coreboot/+/68982/comment/a830a728_ac75f091 
PS2, Line 1: ## SPDX-License-Identifier: GPL-2.0-only
add another newline after the spdx header, to be consistent with most files in the tree; same for all other files and commits

https://review.coreboot.org/c/coreboot/+/68982/comment/47ee2b4f_e9c37657 
PS2, Line 1: ##
one is enough; same for all other files and commits