ifupdown-scripts: do not install .empty files

ifupdown-scripts: do not install .empty files

ifupdown-scripts has some .empty files to maintain empty directories
in git. Previously this package used to be part of the skeleton which
used SYSTEM_RSYNC to copy the directories to the target. When it was
split into a separate package, cp -a was used to do the copy instead,
which copies the .empty files.

Change to SYSTEM_RSYNC which excludes .empty files.

Signed-off-by: Cam Hutchison <camh@xdna.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>