Move GSCLK from PB5 (timer1B) to PB1 (timer2B).

This in preparation for pinout for controlling all three PCBs.
Missing part of previous commit (change of interrupt handler timer1a->timer2a)
Implement PB2 and PB3 as extra GSCLK for pcbs 2 and 3.
Rewrite to support all three blades.

(One blade still works when #define SINGLE_BLADE. All three blades did
not work in first test, not sure if this is due to code or due to
wrong wiring yet.)