package/skeleton: fix message about non-merged custom skeleton

Use of a merged /usr is not restricted to systemd anymore, thus the
current error message is misleading, as it only speaks about systemd.

Fix the message by just ditching the reference to systemd altogether.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>