Make project REUSE compatible

The change was performed as follows:
- it was checked that the COPYING.MIT license was present with the
  initial commit and such each contributor was awere of the license
- mail was around to all previous contributors to check this assumption
  (mail was not replied by Hannah and Sebastian); every reply was a
  confirmation of the assumed MIT licensing
- all copyright information were recovered from the respective Git
  commits
- reuse lint was to used to check the REUSE conformance

classes/kconfig.bbclass

@@ -1,4 +1,8 @@
 # Class for recipes using kconfig host tools
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 DEPENDS += " \
     kconfig \