buildroot

package/systemd: Default yes for timesyncd

It seems logical to enable a few options along with systemd when it's
the init system.

Also change the help as timesyncd is a daemon that implements an SNTP client.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>