Revive power-on charging mode display (charging-sdl)

Charging sdl was in my opinion a pretty neat tool that made postmarketOS feel more feature complete.

I think the existing implementation was somewhat overcomplicated, so why dont we bring it back as something a lot easier:

A shell script (or Go binary if someone fancies) which reads the battery voltage, charge, charging current etc and just displays them on the console, maybe with some cool ascii art or something :D

Rather than using triggerhappy, lets make use of the fact that the power button is configured as a wakeup source, have the script loop for a few second and then disable the backlight and put the system into s2idle (where the CPUs are up but userspace and IO is frozen).

This would be a lot simpler, work out of the box on any device with a framebuffer and battery stats and I think be much more suited to postmarketOS than charging-sdl was.