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>
dhcp: Fix IPv6 files paths

Use IPv6 file locations consistent with those set for IPv4 instead of keeping
the defaults.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>