Add ntp client

[bastelfreak]

we need a working clock for many things, for example correct logs or the installimage. we should set up systemd-timesyncd?

[killermoehre]

Current systemd-timesyncd uses the arch.ntp.org-pool as fallback. Previous versions (and upstream) use the google ntp server for this. We should check this before to make sure not to leak information (yes, there was an discussion about this in #archlinux.de@freenode).

We can set either the global option in timesyncd.conf, or we use a per-interface config. Per-interface is superior if the NTP server is announced via DHCP (using the DHCP announced is the default). But you can always set a static NTP server per interface. We need to decide where to set this setting and which scope it shall have.

[bastelfreak]

IMO we should allow the user to set ntp servers in the config_build.sh, if none are defined we use the default ones provided by the package.