Copter: add pre-arm board voltage check

Copter will not arm unless board voltage is over 4.5V

ArduCopter/config.h

@@ -382,6 +382,10 @@
  # define HIGH_DISCHARGE                 1760
 #endif
 
+#ifndef BOARD_VOLTAGE_MIN
+ # define BOARD_VOLTAGE_MIN             4500        // min board voltage in milli volts for pre-arm checks
+#endif
+
 // Battery failsafe
 #ifndef FS_BATTERY
  # define FS_BATTERY              DISABLED

ArduCopter/motors.pde

@@ -264,6 +264,14 @@ static void pre_arm_checks(bool display_failure)
     }
 #endif
 
+    // check board voltage
+    if(board_voltage() < BOARD_VOLTAGE_MIN) {
+        if (display_failure) {
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Low Board Voltage"));
+        }
+        return;
+    }
+
     // if we've gotten this far then pre arm checks have completed
     ap.pre_arm_check = true;
 }