feat: Power-off fix for the Steam Machine; the kernel pin is optional

Kernels >= 7.2 keep the firmware's S4/S5 wake bit on GPIO pin 18, so the
Steam Machine starts again right after powering off. Steam Machine
support now builds a DKMS module (patches/steamify-fremont-poweroff.c)
that clears it before power-off, for every kernel. The kernel pin is no
longer ticked by default. Module sources and patches move to patches/,
embedded by the single-file build.

patches/README.md

@@ -0,0 +1,10 @@
+# Patches and extra sources
+
+Files Steamify builds or applies, kept out of the shell code so they can be
+read and reviewed on their own. `lib/*.sh` reads them with `patch_file
+<name>`; the single-file build (`.github/tools/bundle.sh`) embeds them.
+
+| File | Used by | What |
+|---|---|---|
+| `steamify-fremont-poweroff.c` | `lib/fremont-poweroff.sh` (Steam Machine support) | Kernel module (DKMS): clears GPIO 18's S4/S5 wake bit before power-off, so the Steam Machine stays off on kernels 7.2 and newer |
+| `cros-ec-cec-single-port.patch` | `lib/cec.sh` (HDMI-CEC) | Makes Valve's Steam Machine CEC driver find amdgpu's HDMI port |