Extend Servo support to work across multiple boards

[Robotmad]

map servos to channels on boards - up to 6x4 = 24 - but we are actually limited to 8 PWM channels.

[Robotmad]

also support motor use across multiple boards - e.g. one motor on each of 2 hexpansions for left/right. or 2 pairs of motors, or 3 motors (omniwheel style?)