LICENSE: all files are licensed under BSD 2-Clause by default

The information contained in the documentation are covered by the BSD
2-Clause license unless stated otherwise. Make this clear by not
mentioning "source files" but simply "files" in LICENSE.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

LICENSE

@@ -1,4 +1,4 @@
-Unless it has its own copyright/license embedded in its body, each source file
+Unless it has its own copyright/license embedded in its body, each file
 is subject to the following license terms:
 
 Copyright (c) 2015, Linaro Limited