dhcp: not override site.h

Previously, site.h was overridden for setting _PATH_DHCPD_CONF
and _PATH_DHCLIENT_CONF, it caused other MACROs were missing,
so we use a patch to instead.

The macros NSUPDATE and COMPACT_LEASES existed in site.h

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>