Make motor subsystem configurable using parameters

Motor pins: MOT_PIN_FL, MOT_PIN_FR, MOT_PIN_RL, MOT_PIN_RR.
PWM configuration: MOT_PWM_FREQ, MOT_PWM_RES, MOT_PWM_STOP, MOT_PWM_MIN, MOT_PWM_MAX.
MOT_PWM_MAX = -1 chooses duty cycle mode for brushed motors (default).

gazebo/flix.h

@@ -32,6 +32,7 @@ void controlRates();
 void controlTorque();
 const char* getModeName();
 void sendMotors();
+int getDutyCycle(float value);
 bool motorsActive();
 void testMotor(int n);
 void print(const char* format, ...);