Some powerd prefs can probably be removed in favor of detecting hardware features at runtime. I should do a full audit, but off the top of my head:

- has_keyboard_backlight: powerd supports hotpluggable keyboard backlights now ( issue 749539 ), and I suspect we can always enable that path now.

- set_wifi_transmit_power_for_tablet_mode: powerd knows if there's a tablet mode switch or not, so I don't know why we need this.

- external_display_only: powerd can detect the presence of an internal display, I think (and it already watches udev events about displays getting connected and disconnected). It knows if it's found an internal backlight or not, although I'm not sure if we should use that to make the decision.

I may be able to delete some USE flags as part of this as well. I need to be careful since I believe that some hardware doesn't appear immediately at boot -- I've seen this happen with ambient light sensors, for instance, and powerd currently defers making its initial backlight brightness adjustment if it expects an ALS to show up eventually.