package/sysklogd: fix installation path of the daemons

Use "--exec-prefix=/" to install syslogd and klogd at /sbin, as required
by the init scripts. This also ensures that the BusyBox counterparts are
not installed.

Update the systemd unit files, accordingly.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>