message: Adds license header to hal.h
diff: -#include <stdint.h>
  +/* Copyright 2020, RespiraWorks
  +
  +Licensed under the Apache License, Version 2.0 (the "License");
  +you may not use this file except in compliance with the License.
  +You may obtain a copy of the License at
  +
  + http://www.apache.org/licenses/LICENSE-2.0
  +
  +Unless required by applicable law or agreed to in writing, software
  +distributed under the License is distributed on an "AS IS" BASIS,
  +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  +See the License for the specific language governing permissions and
  +limitations under the License.
  +*/
  // A hardware abstraction layer that supports mocking/faking for tests.
  //
  // they come at a cost. Thus we do some macro magic to get virtual methods
  // when compiling for test, but not when compiling for production.
  +#ifndef HAL_H
  +#define HAL_H
  +
  +#include <stdint.h>
  +
  #ifdef TEST_MODE
  #ifdef AVR
  @@ -137,3 +155,5 @@ inline void HalApi::digitalWrite(int pin, VoltageLevel value) {
  }
  #endif
  +
  +#endif // HAL_H
  