system/skeleton: fix /etc/group

When using busybox compiled from current git, login emits the following
message:

    login: /etc/group: bad record

Fix it by adding the missing colon to /etc/group in system/skeleton.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>