Kconfig: describe NET, NO_NET, LWIP_DEBUG and LWIP_ASSERT

Some Kconfig symbols introduced in commit 8cb330355bd5 ("net: introduce
alternative implementation as net/lwip/") need a full description. The
NET symbol needs one, too.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/Kconfig b/Kconfig
index eb55f25..1f5b0f1 100644
--- a/Kconfig
+++ b/Kconfig
@@ -765,10 +765,15 @@
 
 config NO_NET
 	bool "No networking support"
+	help
+	  Do not include networking support
 
 config NET
 	bool "Legacy U-Boot networking stack"
 	imply NETDEVICES
+	help
+	  Include networking support with U-Boot's internal implementation of
+	  the TCP/IP protocol stack.
 
 config NET_LWIP
 	bool "Use lwIP for networking stack"