cerbero: Cache the output of shell.ls_files
cerbero: Cache the output of shell.ls_files

Sometimes we call it on the same files multiple times. Speeds up
recipe parsing + packaging from 5.0s to 4.8s.

GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,fluendo/cerbero,GStreamer/cerbero,atsushieno/cerbero,fluendo/cerbero,GStreamer/cerbero,nirbheek/cerbero,atsushieno/cerbero,atsushieno/cerbero,fluendo/cerbero,atsushieno/cerbero,GStreamer/cerbero,nirbheek/cerbero,fluendo/cerbero,atsushieno/cerbero,fluendo/cerbero