DietPi-Config | RPi: Disable HDMI + framebuffer completely via config.txt entries

[MichaIng]

Ref: raspberrypi/userland#447

Should be preferred, instead of manually running tvservice -o on boot. Also framebuffer is disabled completely, so does not need to be reduced.
The downside is, that there is no chance to re-enable HDMI without a reboot, but with our framebuffer x=y=16 this was anyway practically the case.

PR up, which as well removes some other obsolete settings: https://github.com/Fourdee/DietPi/pull/2402

[Fourdee]

@MichaIng

Good work, tested on RPi with HDMI disabled, no output to HDMI TV. Will mark as completed 👍