system: set root password only for default skeleton

In case one is using a custom skeleton, the root pasword might already be
set in this case, and should not be overriden.

Just ask for (and set) the root password only for the default skeleton.

Reported-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>