gitignore: ignore bitbake documentation build products
Missed the bitbake manual the last time around because it's in a
different directory.
Signed-off-by: Sascha Silbe <x-yo17@se-silbe.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>