net: lwip: dhcp: support arguments for TFTP file download

The dhcp command is supposed to have the following syntax as per
"help dhcp":

  dhcp [loadAddress] [[hostIPaddr:]bootfilename]

In other words, any arguments should be passed to an implicit
tftpboot command after the DHCP exchange has occurred.

Add the missing code to the lwIP version of do_dhcp().

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/net/lwip/dhcp.c b/net/lwip/dhcp.c
index bfc72ca..9b882cf 100644
--- a/net/lwip/dhcp.c
+++ b/net/lwip/dhcp.c
@@ -111,9 +111,21 @@
 
 int do_dhcp(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
+	int ret;
+
 	eth_set_current();
 
-	return dhcp_loop(eth_get_dev());
+	ret = dhcp_loop(eth_get_dev());
+	if (ret)
+		return ret;
+
+	if (argc > 1) {
+		struct cmd_tbl cmdtp = {};
+
+		return do_tftpb(&cmdtp, 0, argc, argv);
+	}
+
+	return CMD_RET_SUCCESS;
 }
 
 int dhcp_run(ulong addr, const char *fname, bool autoload)