AP_HAL: added wait_pin() API

for waiting on data-ready lines

libraries/AP_HAL/GPIO.h

@@ -65,6 +65,12 @@ public:
         return attach_interrupt(pin, (AP_HAL::Proc)nullptr, AP_HAL::GPIO::INTERRUPT_NONE);
     }
 
+    /*
+      block waiting for a pin to change. A timeout of 0 means wait
+      forever. Return true on pin change, false on timeout
+     */
+    virtual bool wait_pin(uint8_t pin, INTERRUPT_TRIGGER_TYPE mode, uint32_t timeout_us) { return false; }
+
     /* return true if USB cable is connected */
     virtual bool    usb_connected(void) = 0;
 };