And reorder other XPS 13.

pcengines/apu: expose GRUB over serial

Prior to this commit, an apu's boot would go through stages of varying visibility:
1. BIOS: exposed to serial
2. GRUB: not exposed to serial
3. Linux: exposed to serial

This commit changes (2) to be visible over serial, so the entire boot is visible over serial.

Tested on nixos 18.09 on a https://www.pcengines.ch/apu1d.htm

lenovo-x1: removed battery TLP threshholds

This is pretty much a copy of XPS 9370, without kaby-laky and the throttle bug. I was getting lockups with the kaby-laky changes and the throttle bug did not seem to have an impact either way.

add dummy default.nix

fix nix-env evaluation

nix-env picked up `default.nix`, which contained our hardware profiles.
This is not only cpu-intensive to evaluate but also fails because
it does some import-from-derivation.
By moving default.nix to release.nix it should be no longer loaded

lenovo x1 carbon gen6: update S3 suspend

With UEFI >= 1.30, there's an explicit option to enable S3 power management.
Once this is selected, S3 is enabled and "deep" is selected as the default
/sys/power/mem_sleep value without requiring any kernel boot parameters.

This adds basic support for the Thinkpad T440s, based on the work done for the T440p. The expression enables:

- microcode updates
- trackpoint support
- kernel module for battery level management

common/cpu: default microcode updates to enableRedistributableFirmware

copy of #81

lenovo/thinkpad/t450s: init

- BeagleBoard PocketBeagle
- PC Engines APU
- Purism Librem 13v3

thinkpad: introduce t430

Copied everything from t440p, added a fix for the brightness keys

lenovo/thinkpad/x260: Init

Fix unusable Lenovo Thinkpad X1 profile

As proposed by @garbas in #60

[rebased from #67] Suggestions for CONTRIBUTING