t420 can be reprogrammed through test points under palmrest

[tlaurion]

Might be interesting to add that info, with screenshots, from board owners?
Info from https://matrix.to/#/!dmTEygsokeASkUDDyG:matrix.org/$-PhJiCBBi5nNb8qXVVEi1tEmp9rga3aLWi2_1v1xpYQ?via=matrix.org&via=sibnsk.net&via=mozilla.org

Tagging the willing ones from linuxboot/heads#692:
@alexmaloteaux @natterangell (iGPU) @akfhasodh @doob85

[akoutian]

The via hole layout for the T420 is shown in the following picture:

These vias can be accessed by removing the palmrest and lifting the display, magnesium frame and motherboard assembly out of the chassis. It is not necessary to separate the motherboard from the frame, and thus no removal of the fan assembly or repasting is needed.