Convert CONFIG_UDP_CHECKSUM to Kconfig

This converts the following to Kconfig:
   CONFIG_UDP_CHECKSUM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Kconfig b/net/Kconfig
index 8e9579a..8ddea28 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -108,4 +108,11 @@
 	  enabled, when an ARP reply is received, the server's IP address is
 	  written there.
 
+config UDP_CHECKSUM
+	bool "Check the UDP checksum"
+	help
+	  Enable this to verify the checksum on UDP packets. If the checksum
+	  is wrong then the packet is discussed and an error is shown, like
+	  "UDP wrong checksum 29374a23 30ff3826"
+
 endif   # if NET