Change in coreboot[master]: mb/hp/z220_series: Convert z220_sff_workstation into variant

[Felix Singer (Code Review)]

Attention is currently required from: Damien Zammit, Edward O'Callaghan, Angel Pons.
Felix Singer has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/62808 )

Change subject: mb/hp/z220_series: Convert z220_sff_workstation into variant
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Patch Set 2:

(1 comment)

File src/mainboard/hp/z220_series/Kconfig:

https://review.coreboot.org/c/coreboot/+/62808/comment/87c1f3ae_14c4e724 
PS2, Line 1: ## SPDX-License-Identifier: GPL-2.0-only
> Yes, please remove. We don't use license headers in Kconfig files anymore.

Hmm.. Looking at the tree, there are many Kconfig files with license headers. Though, not all Kconfig files have one. I thought we removed them from Kconfig files.