Support 'forward only' motor control mode

[Ezward]

Currently each wheel uses two pwm pins, one for forward and one for backward. This is good because it (a) allows us to backup (b) allows us to turn in place. However, if we only support a forward pwm pin on each motor, then we would have two free pins. This would then allow us to read the encoders with those pins. Then we could support use of the serial ports without having to disconnect that encoders.

• support compile-time optional drive train mode where we only supply forward pwm to each wheel.
• when the firmware is compiled in this mode, the Turtle Control web UI must hide the 'Reverse' button because the rover cannot drive in reverse.
• when the firmware is compiled in forward-only mode the Go to goal must use a slightly different 'Turn to Angle' algorithm.
• when the firmware is compiled in forward-only mode, the Game Controller support in the web UI must use a slightly different algorithm that does not allow for reverse. Perhaps simply setting any reverse value to zero is adequate.
• when the firmware is compiled in forward-only mode, allow the serial port to be activated even if encoders are activated. The encoder pins will be reassigned to the former reverse pwm pins in this mode.