Update license headers

src/arch/x86_64/boot.asm

@@ -1,16 +1,11 @@
-; Copyright 2015 Philipp Oppermann
+; Copyright 2015 Philipp Oppermann. See the README.md
+; file at the top-level directory of this distribution.
 ;
-; 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.
+; Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+; http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+; <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+; option. This file may not be copied, modified, or distributed
+; except according to those terms.
 
 global start
 extern long_mode_start