toaster.bbclass: read list of files in image

We read the list of files in a built image and send it
over with the same event for packages in image.

(From OE-Core rev: 21bb659beca69c8bb379af2bf10afc843f529e57)

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>