When `lcd_fill(0x00)` is called before drawing the splash screen and
only a part of the image is stored as done now, some memory in the
`.bss` segment can be used for something else.
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%.
