class/lib: Fix up various file access methods

class/lib: Fix up various file access methods

There are various bits of cruft that have built up around our file accesses. This patch
cleans some of them up, specifically:

 * Remove pointless "from __builtin__ import file"
 * Use open(), not file()
 * Wrap file usage in a with container to ensure files are closed
 * Add missing .close() calls in some cases

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>