Reformat and update copyright and license notices

For everything before 2015 an OAR copyright is assumed.  Later copyright
notices are based on Git commits on a best effort basis.

The copyright notices of the "RTEMS Software Engineering Handbook" and
the "RTEMS CPU Architecture Supplement" are incomplete.

common/license.rst

@@ -0,0 +1,6 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2019 embedded brains GmbH
+
+This document is available under the
+`Creative Commons Attribution-ShareAlike 4.0 International Public License <https://creativecommons.org/licenses/by-sa/4.0/legalcode>`_.