[DHCPD] add new version DHCPD use lwip raw api.

components/net/lwip_dhcpd/SConscript

 from building import *
 
 cwd = GetCurrentDir()
-src = Glob('*.c')
+
+if GetDepend('RT_USING_LWIP202'):
+    src = ['dhcp_server_raw.c']
+else:
+    src = ['dhcp_server.c']
 
 CPPPATH = [cwd]
 

components/net/lwip_dhcpd/dhcp_server.h

@@ -31,7 +31,15 @@
 #ifndef DHCPV4_SERVER_H__
 #define DHCPV4_SERVER_H__
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void dhcpd_start(const char *netif_name);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif