Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Kconfig b/net/Kconfig
index 8ddea28..e5b5837 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -110,6 +110,7 @@
 
 config UDP_CHECKSUM
 	bool "Check the UDP checksum"
+	default y if SANDBOX
 	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