APM: enable Follow mode for rovers/boats

this works with ArduPilot 4.1/4.2

src/FirmwarePlugin/APM/ArduRoverFirmwarePlugin.cc

@@ -22,10 +22,7 @@ APMRoverMode::APMRoverMode(uint32_t mode, bool settable)
         {STEERING,       "Steering"},
         {HOLD,           "Hold"},
         {LOITER,         "Loiter"},
-#if 0
-    // Follow me not ready for Stable
         {FOLLOW,         "Follow"},
-#endif
         {SIMPLE,         "Simple"},
         {AUTO,           "Auto"},
         {RTL,            "RTL"},
@@ -43,10 +40,7 @@ ArduRoverFirmwarePlugin::ArduRoverFirmwarePlugin(void)
         APMRoverMode(APMRoverMode::STEERING     ,true),
         APMRoverMode(APMRoverMode::HOLD         ,true),
         APMRoverMode(APMRoverMode::LOITER       ,true),
-#if 0
-    // Follow me not ready for Stable
         APMRoverMode(APMRoverMode::FOLLOW       ,true),
-#endif
         APMRoverMode(APMRoverMode::SIMPLE       ,true),
         APMRoverMode(APMRoverMode::AUTO         ,true),
         APMRoverMode(APMRoverMode::RTL          ,true),