AP_BoardConfig: added BRD_ALT_CONFIG parameter

allow for alternative hw configs to be selected

libraries/AP_BoardConfig/AP_BoardConfig.cpp

@@ -279,6 +279,15 @@ const AP_Param::GroupInfo AP_BoardConfig::var_info[] = {
     // @User: Advanced
     AP_SUBGROUPINFO(heater.pi_controller, "IMUHEAT_",  21, AP_BoardConfig, AC_PI),
 #endif
+
+    // @Param: ALT_CONFIG
+    // @DisplayName: Alternative HW config
+    // @Description: Select an alternative hardware configuration. A value of zero selects the default configuration for this board. Other values are board specific. Please see the documentation for your board for details on any alternative configuration values that may be available.
+    // @Range: 0 10
+    // @Increment: 1
+    // @User: Advanced
+    // @RebootRequired: True
+    AP_GROUPINFO("ALT_CONFIG", 22, AP_BoardConfig, _alt_config, 0),
     
     AP_GROUPEND
 };

libraries/AP_BoardConfig/AP_BoardConfig.h

@@ -116,6 +116,11 @@ public:
         return _singleton?_singleton->pwm_count.get():8;
     }
 
+    // get alternative config selection
+    uint8_t get_alt_config(void) {
+        return uint8_t(_alt_config.get());
+    }
+    
     enum board_safety_button_option {
         BOARD_SAFETY_OPTION_BUTTON_ACTIVE_SAFETY_OFF= (1 << 0),
         BOARD_SAFETY_OPTION_BUTTON_ACTIVE_SAFETY_ON=  (1 << 1),
@@ -251,6 +256,8 @@ private:
     AP_Int16 _boot_delay_ms;
 
     AP_Int32 _options;
+
+    AP_Int8  _alt_config;
 };
 
 namespace AP {