AP_Notify: add RCOutputRGBLedInverted

Some RGB leds controlled with an inverted logic. This class addresses
this.

libraries/AP_Notify/RCOutputRGBLed.cpp

@@ -53,6 +53,17 @@ bool RCOutputRGBLed::hw_init()
     return true;
 }
 
+uint16_t RCOutputRGBLed::get_duty_cycle_for_color(const uint8_t color, const uint16_t usec_period) const
+{
+    return  usec_period * color / _led_bright;
+}
+
+uint16_t RCOutputRGBLedInverted::get_duty_cycle_for_color(const uint8_t color, const uint16_t usec_period) const
+{
+    return  usec_period * (255 - color) / _led_bright;
+}
+
+
 bool RCOutputRGBLed::hw_set_rgb(uint8_t red, uint8_t green, uint8_t blue)
 {
     const uint16_t freq_motor = hal.rcout->get_freq(0);
@@ -69,13 +80,13 @@ bool RCOutputRGBLed::hw_set_rgb(uint8_t red, uint8_t green, uint8_t blue)
         hal.rcout->set_freq(mask, freq_motor);
     }
 
-    uint16_t usec_duty = usec_period * red / _led_bright;
+    uint16_t usec_duty = get_duty_cycle_for_color(red, usec_period);
     SRV_Channels::set_output_pwm_chan(_red_channel, usec_duty);
 
-    usec_duty = usec_period * green / _led_bright;
+    usec_duty = get_duty_cycle_for_color(green, usec_period);
     SRV_Channels::set_output_pwm_chan(_green_channel, usec_duty);
 
-    usec_duty = usec_period * blue / _led_bright;
+    usec_duty = get_duty_cycle_for_color(blue, usec_period);
     SRV_Channels::set_output_pwm_chan(_blue_channel, usec_duty);
 
     return true;

libraries/AP_Notify/RCOutputRGBLed.h

@@ -13,6 +13,7 @@ public:
 protected:
     bool hw_init() override;
     virtual bool hw_set_rgb(uint8_t red, uint8_t green, uint8_t blue) override;
+    virtual uint16_t get_duty_cycle_for_color(const uint8_t color, const uint16_t usec_period) const;
 
 private:
     uint8_t _red_channel;
@@ -20,6 +21,15 @@ private:
     uint8_t _blue_channel;
 };
 
+class RCOutputRGBLedInverted : public RCOutputRGBLed {
+public:
+    RCOutputRGBLedInverted(uint8_t red_channel, uint8_t green_channel, uint8_t blue_channel)
+        : RCOutputRGBLed(red_channel, green_channel, blue_channel)
+    { }
+protected:
+    virtual uint16_t get_duty_cycle_for_color(const uint8_t color, const uint16_t usec_period) const override;
+};
+
 class RCOutputRGBLedOff : public RCOutputRGBLed {
 public:
     RCOutputRGBLedOff(uint8_t red_channel, uint8_t green_channel,