ntp: allow to run pkg_postinst_ntpdate in do_rootfs

* read-only rootfs cannot run postinsts on first boot and in this case
  it doesn't really have to as we can create cron task in do_rootfs

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe.macdonald@windriver.com>