Support W520

Is your feature request related to a problem? Please describe.
t420 and x220 are supported through Heads. But not the w520.

Describe the solution you'd like
Support w520. For that, board owners need to engage in testing, and participate in porting/testing of the images, with external reprogrammer and willingness to test the port until merged and upon coreboot version changes.

Describe alternatives you've considered
Not using Heads

Additional context
Port for t420 should give insight on what is the process of adding another xx20 platform.

That means

• copying and specializing board and directory (this is what is used to call make BOARD=w520-maximized)
• copying and specializing config/coreboot-t420-maximized.config into config/coreboot-t520-maximized.config (this means changing directories pointing to t520 to w520, making sure dGPU blobs for t420 exist/are needed etc)
• if dGPU blobs are needed, then duplication of blobs/dgpu-w520 will be needed (nvidia card in there?)
• testing.
• ideally creating a PR that is tested by board owner.
• Adding oneself as volonteer for testing under #692

Start by comparing the existing t420 board and directory with the proposed w520 port, then inspect config/coreboot-t420-maximized.config and the corresponding w520 configuration. Check whether dGPU blobs are needed, copy or specialize the relevant board and blob paths, and test the images with an external reprogrammer. Done means a tested pull request with board-owner participation and a testing volunteer listed under #692.