AP_Notify: add AP_NOTIFY_NEOPIXEL_MASK define support

Allow boards to specify a default LED channel mask via
AP_NOTIFY_NEOPIXEL_MASK define, enabling NeoPixel output without
requiring explicit servo function assignment.

Author: andyp1per

libraries/AP_Notify/AP_Notify_config.h

@@ -128,6 +128,10 @@
 #define AP_NOTIFY_NEOPIXEL_ENABLED AP_NOTIFY_SERIALLED_ENABLED
 #endif
 
+#ifndef AP_NOTIFY_NEOPIXEL_MASK
+#define AP_NOTIFY_NEOPIXEL_MASK 0
+#endif
+
 #ifndef AP_NOTIFY_TONEALARM_ENABLED
 #define AP_NOTIFY_TONEALARM_ENABLED 0
 #endif

libraries/AP_Notify/NeoPixel.cpp

@@ -50,6 +50,13 @@ uint16_t NeoPixel::init_ports()
         mask |= SRV_Channels::get_output_channel_mask(fn);
     }
 
+#if AP_NOTIFY_NEOPIXEL_MASK
+    // Use board-defined default mask if no servo functions assigned
+    if (mask == 0) {
+        mask = AP_NOTIFY_NEOPIXEL_MASK;
+    }
+#endif
+
     if (mask == 0) {
         return 0;
     }