AP_BoardConfig: add Board option to disable MAVftp in low mem bds/apps

libraries/AP_BoardConfig/AP_BoardConfig.cpp

@@ -270,7 +270,7 @@ const AP_Param::GroupInfo AP_BoardConfig::var_info[] = {
     // @Param: OPTIONS
     // @DisplayName: Board options
     // @Description: Board specific option flags
-    // @Bitmask: 0:Enable hardware watchdog
+    // @Bitmask: 0:Enable hardware watchdog, 1:Disable MAVftp
     // @User: Advanced
     AP_GROUPINFO("OPTIONS", 19, AP_BoardConfig, _options, HAL_BRD_OPTIONS_DEFAULT),
 

libraries/AP_BoardConfig/AP_BoardConfig.h

@@ -164,8 +164,14 @@ public:
 
     enum board_options {
         BOARD_OPTION_WATCHDOG = (1 << 0),
+        DISABLE_FTP = (1<<1),
     };
 
+    // return true if ftp is disabled
+    static bool ftp_disabled(void) {
+        return _singleton?(_singleton->_options & DISABLE_FTP)!=0:1;
+    }
+
     // return true if watchdog enabled
     static bool watchdog_enabled(void) {
         return _singleton?(_singleton->_options & BOARD_OPTION_WATCHDOG)!=0:HAL_WATCHDOG_ENABLED_DEFAULT;