spdx: add spdx tags to most files

Large change that removes the duplicated MIT notice withe a spdx tag

Signed-off-by: Thomas Ingleby <thomas.ingleby@intel.com>

api/mraa.hpp

@@ -20,6 +20,8 @@
  * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * SPDX-License-Identifier: MIT
  */
 
 #pragma once