dhcp: Fix config files paths

The built dhcp binaries and the installed startup scripts used configuration
files under /etc/, but the configuration files were installed under /etc/dhcp/.
Fix everything to have configuration files under /etc/dhcp/.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>