isc-dhcp: make files/ consistent by flattening

Some /etc/config-destined files are in files/ and suffixed with
.conf while others are in files/etc/config/ which isn't consistent.
Put everything in files/ and call it good.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

net/isc-dhcp/Makefile

@@ -196,7 +196,7 @@ define Package/isc-dhcp-relay-$(BUILD_VARIANT)/install
 	$(INSTALL_DIR) $(1)/usr/sbin
 	$(INSTALL_BIN) $(PKG_INSTALL_DIR)/usr/sbin/dhcrelay $(1)/usr/sbin
 	$(INSTALL_DIR) $(1)/etc/config
-	$(INSTALL_DATA) ./files/etc/config/dhcrelay $(1)/etc/config
+	$(INSTALL_DATA) ./files/dhcrelay.conf $(1)/etc/config
 	$(INSTALL_DIR) $(1)/etc/init.d
 	$(INSTALL_BIN) ./files/dhcrelay4.init $(1)/etc/init.d/dhcrelay4
 endef