Added some debug messages and DHCP test code

src/tests/dhcptest.c

@@ -0,0 +1,10 @@
+#include <string.h>
+#include <gpxe/dhcp.h>
+
+int test_dhcp ( struct net_device *netdev ) {
+	struct dhcp_session dhcp;
+
+	memset ( &dhcp, 0, sizeof ( dhcp ) );
+	dhcp.netdev = netdev;
+	return async_wait ( start_dhcp ( &dhcp ) );
+}