[S] Change in coreboot[master]: Docs: Add SPDX headers to Makefiles

[Angel Pons (Code Review)]

Attention is currently required from: Felix Singer, Martin L Roth.

Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/68957 )

Change subject: Docs: Add SPDX headers to Makefiles
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Patch Set 2: Code-Review+2

(2 comments)

File Documentation/Makefile:

https://review.coreboot.org/c/coreboot/+/68957/comment/f1b750ef_949725d1 
PS2, Line 1: ## SPDX-License-Identifier: GPL-2.0-only
nit: Using one # would be consistent with the existing comments.

File Documentation/Makefile.sphinx:

https://review.coreboot.org/c/coreboot/+/68957/comment/73498b69_d7892728 
PS2, Line 1: ## SPDX-License-Identifier: GPL-2.0-only
Ditto