Configure fuses for the `fuses` target in Makefile to disable
`CKDIV8`. Adapt PWM generation in `lcd_update_backlight()` to new
clock division factor, as well as interrupt handling, debouncing and
dechattering.
This also enables partially lower PWM frequencies (between off and
former lowest PWM setting) for a dimmer lcd backlight.
To have the values of `value_contrast` and `value_backlight` persist
over turn offs they are loaded and stored from and in the eeprom.
For preventing the flash target from earasing the eeprom the `EESAVE`
high fuse byte is set to 0 (programmed).
The values are now loaded when booted and stored when a new value is
set in the setup menu.
Enable the SPI bus to configure the display and program its ram. For
that the fastest available SPI clock is used. To configure the display
a minimal config is used which is not identical but similar to the
displays datasheet. The display can only be filled columnwise so far.
Generate a variable duty cycle PWM signal for the dimmable display
backlight at a frequency of 1.25kHz.
For demo und testing purposes PD5 is currently fading between 0 and
100%.
