AP_GPS: create and use AP_GPS_BACKEND_DEFAULT_ENABLED

Allows for all backends to be set to off by default.

libraries/AP_GPS/AP_GPS_ERB.h

@@ -25,7 +25,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_ERB_ENABLED
-  #define AP_GPS_ERB_ENABLED 1
+  #define AP_GPS_ERB_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_ERB_ENABLED

libraries/AP_GPS/AP_GPS_GSOF.h

@@ -23,7 +23,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_GSOF_ENABLED
-  #define AP_GPS_GSOF_ENABLED 1
+  #define AP_GPS_GSOF_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_GSOF_ENABLED

libraries/AP_GPS/AP_GPS_MAV.h

@@ -26,7 +26,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_MAV_ENABLED
-  #define AP_GPS_MAV_ENABLED 1
+  #define AP_GPS_MAV_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif 
 
 #if AP_GPS_MAV_ENABLED

libraries/AP_GPS/AP_GPS_NMEA.h

@@ -47,7 +47,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_NMEA_ENABLED
-  #define AP_GPS_NMEA_ENABLED 1
+  #define AP_GPS_NMEA_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_NMEA_ENABLED

libraries/AP_GPS/AP_GPS_NOVA.h

@@ -23,7 +23,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_NOVA_ENABLED
-  #define AP_GPS_NOVA_ENABLED 1
+  #define AP_GPS_NOVA_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_NOVA_ENABLED

libraries/AP_GPS/AP_GPS_SBF.h

@@ -23,7 +23,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_SBF_ENABLED
-  #define AP_GPS_SBF_ENABLED 1
+  #define AP_GPS_SBF_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_SBF_ENABLED

libraries/AP_GPS/AP_GPS_SBP.h

@@ -25,7 +25,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_SBP_ENABLED
-  #define AP_GPS_SBP_ENABLED 1
+  #define AP_GPS_SBP_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_SBP_ENABLED

libraries/AP_GPS/AP_GPS_SBP2.h

@@ -25,7 +25,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_SBP2_ENABLED
-   #define AP_GPS_SBP2_ENABLED 1
+   #define AP_GPS_SBP2_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_SBP2_ENABLED

libraries/AP_GPS/AP_GPS_SIRF.h

@@ -26,7 +26,7 @@
 #include "GPS_Backend.h"
 
 #ifndef AP_GPS_SIRF_ENABLED
-  #define AP_GPS_SIRF_ENABLED 1
+  #define AP_GPS_SIRF_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
 
 #if AP_GPS_SIRF_ENABLED

libraries/AP_GPS/GPS_Backend.h

@@ -22,6 +22,10 @@
 #include <AP_RTC/JitterCorrection.h>
 #include "AP_GPS.h"
 
+#ifndef AP_GPS_BACKEND_DEFAULT_ENABLED
+#define AP_GPS_BACKEND_DEFAULT_ENABLED 1
+#endif
+
 #ifndef AP_GPS_DEBUG_LOGGING_ENABLED
 // enable this to log all bytes from the GPS. Also needs a call to
 // log_data() in each backend