dhcp: install dhclient to /sbin

Debian ifupdown uses an explicit call to /sbin/dhclient.
Busybox ifupdown uses the search path.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>