bbconfigbuild/configfragments.py: replace fragment_exists() with a more generic get_f...

bbconfigbuild/configfragments.py: replace fragment_exists() with a more generic get_fragment()

This can be used to get fragment information by its name (or None,
if there's no fragment), rather than just check for its existence.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>