bitbake/cooker.py: Ensure BBFILES is processed in order

bitbake/cooker.py: Ensure BBFILES is processed in order

The files found by collect_bbfiles should be processed in order but due
to being processed using python's set(), the order was not being preserved.

Use a list instead as whilst the code is slightly more ugly, order
is preserved.

Addresses [YOCTO #1100]

Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>