sntp: use udp framework

This commits update the support of sntp to use
the framework udp. This change allows to remove
all the reference to sntp in the main network
file net/net.c.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 30c358d..999b6cf 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1645,6 +1645,7 @@
 
 config CMD_SNTP
 	bool "sntp"
+	select PROT_UDP
 	help
 	  Synchronize RTC via network