Documentation: Add markdown license information

Add CC-by 4.0 license written in markdown and add global copyright string.

Change-Id: I31dc540d63b289d38d6d7d9a7c35b6bb50f9c92d
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/26264
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/conf.py

@@ -11,7 +11,7 @@ master_doc = 'index'
 
 # General information about the project.
 project = u'coreboot'
-copyright = u'the coreboot project'
+copyright = u'CC-by 4.0 the coreboot project'
 author = u'the coreboot project'
 
 # The version info for the project you're documenting, acts as replacement for