SRV_Channel: allow build with no channels

needed for AP_Periph when using LED driver only

libraries/SRV_Channel/SRV_Channel.h

@@ -22,11 +22,15 @@
 #include <AP_SBusOut/AP_SBusOut.h>
 #include <AP_BLHeli/AP_BLHeli.h>
 
-#if !defined(NUM_SERVO_CHANNELS) && defined(HAL_BUILD_AP_PERIPH) && defined(HAL_PWM_COUNT) && (HAL_PWM_COUNT >= 1)
+#ifndef NUM_SERVO_CHANNELS
+#if defined(HAL_BUILD_AP_PERIPH) && defined(HAL_PWM_COUNT)
     #define NUM_SERVO_CHANNELS HAL_PWM_COUNT
+#elif defined(HAL_BUILD_AP_PERIPH)
+    #define NUM_SERVO_CHANNELS 0
 #else
     #define NUM_SERVO_CHANNELS 16
 #endif
+#endif
 
 class SRV_Channels;
 
@@ -475,7 +479,11 @@ public:
     static AP_HAL::RCOutput::DshotEscType get_dshot_esc_type() { return AP_HAL::RCOutput::DshotEscType(_singleton->dshot_esc_type.get()); }
 
     static SRV_Channel *srv_channel(uint8_t i) {
+#if NUM_SERVO_CHANNELS > 0
         return i<NUM_SERVO_CHANNELS?&channels[i]:nullptr;
+#else
+        return nullptr;
+#endif
     }
 
     // SERVO* parameters

libraries/SRV_Channel/SRV_Channel_aux.cpp

@@ -21,6 +21,10 @@
 #include <AP_HAL/AP_HAL.h>
 #include <RC_Channel/RC_Channel.h>
 
+#if NUM_SERVO_CHANNELS == 0
+#pragma GCC diagnostic ignored "-Wtype-limits"
+#endif
+
 extern const AP_HAL::HAL& hal;
 
 /// map a function to a servo channel and output it

libraries/SRV_Channel/SRV_Channels.cpp

@@ -34,6 +34,10 @@
   #include <AP_PiccoloCAN/AP_PiccoloCAN.h>
 #endif
 
+#if NUM_SERVO_CHANNELS == 0
+#pragma GCC diagnostic ignored "-Wtype-limits"
+#endif
+
 extern const AP_HAL::HAL& hal;
 
 SRV_Channel *SRV_Channels::channels;