[DO NOT MERGE] px4_i2c_device_external hacks

platforms/common/include/px4_platform_common/i2c.h

@@ -66,6 +66,11 @@ static inline bool px4_i2c_bus_external(int bus)
 	return true;
 }
 
+/**
+ * runtime-check if a board has a specific device as external.
+ * This can be overridden by a board to add run-time checks.
+ */
+__EXPORT bool px4_i2c_device_external(const uint32_t device_id);
 
 /**
  * @class I2CBusIterator