AP_Arming: add mandatory_checks

These are checks that can never be bypassed

libraries/AP_Arming/AP_Arming.cpp

@@ -872,7 +872,7 @@ bool AP_Arming::arm(AP_Arming::Method method, const bool do_arming_checks)
         return false;
     }
 
-    if (!do_arming_checks || (pre_arm_checks(true) && arm_checks(method))) {
+    if ((!do_arming_checks && mandatory_checks(true)) || (pre_arm_checks(true) && arm_checks(method))) {
         armed = true;
 
         //TODO: Log motor arming

libraries/AP_Arming/AP_Arming.h

@@ -133,6 +133,9 @@ protected:
     bool servo_checks(bool report) const;
     bool rc_checks_copter_sub(bool display_failure, const RC_Channel *channels[4]) const;
 
+    // mandatory checks that cannot be bypassed.  This function will only be called if ARMING_CHECK is zero or arming forced
+    virtual bool mandatory_checks(bool report) { return true; }
+
     // returns true if a particular check is enabled
     bool check_enabled(const enum AP_Arming::ArmingChecks check) const;
     // returns a mavlink severity which should be used if a specific check fails