Makefile: don't depend on current skeleton/overlay permissions

We use 'rsync -a' to copy the skeleton and overlays, so the target ends
up with the exact same permissions as on the repo. The problem is we
don't track these permissions, since Git doesn't allow for that (except
for the exec bit). This means users with different umasks at the time of
cloning could end up with different target permissions.

Fix this by using --chmod on rsync calls so we don't depend on the
current permission set for the skeleton and overlays. We do depend on
the exec bit, but that's fine since that one is tracked by Git.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>