net: Rename SPL_NET_SUPPORT to SPL_NET

Rename this option so that CONFIG_IS_ENABLED can be used with it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/SPL/README.am335x-network b/doc/SPL/README.am335x-network
index e052706..9599729 100644
--- a/doc/SPL/README.am335x-network
+++ b/doc/SPL/README.am335x-network
@@ -7,7 +7,7 @@
 
  I. Building the required images
   1. You have to enable generic SPL configuration options (see
-doc/README.SPL) as well as CONFIG_SPL_NET_SUPPORT,
+doc/README.SPL) as well as CONFIG_SPL_NET,
 CONFIG_SPL_ETH, CONFIG_SPL_LIBGENERIC_SUPPORT and
 CONFIG_SPL_LIBCOMMON_SUPPORT in your board configuration file to build
 SPL with support for booting over the network. Also you have to enable