The functionality of timer0 and timer2 are switched between ATmega64/128
and 641/1281/2561. When compiled for ATmega2561, this commit sets up timer0
in the same way as timer2 on ATmega64/128 (phase correct PWM, clk/64 count
rate, inverted if SP22 is defined).
The previous quick fix had a typo in the initialization of TCCR0B (0b111 is
an invalid value for the clock select bits CS2:0). It also did not respect
the SP22 define which indicates the PWM must be inverted. Smartieparts
v2.2 or later programmer boards have an inverted backlight driver. Finally,
it is unnecessary to set DDB7 as it is already set in boardInit() (line
101).
