toaster.bbclass: only scan files-in-image.txt if it exists
We can reach the method in toaster.bbclass which tries to read from
the files-in-image.txt file via a build which doesn't create that
file (e.g. "bitbake core-image-minimal -c rootfs"). This causes
the build to fail with an exception.
Check that this file exists before trying to read from it.
[YOCTO #9784]
Signed-off-by: Elliot Smith <elliot.smith@intel.com> Signed-off-by: bavery <brian.avery@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
