Added a few more files to gitignore file to allow document
generation testing.

(Bitbake rev: f6310ca9ccb402a7569a79a4e92751c6d5733697)

Signed-off-by: Bill Traynor <wmat@alphatroop.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>