common: Drop net.h from common header

Move this header out of the common header. Network support is used in
quite a few places but it still does not warrant blanket inclusion.

Note that this net.h header itself has quite a lot in it. It could be
split into the driver-mode support, functions, structures, checksumming,
etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/compulab/cl-som-imx7/cl-som-imx7.c b/board/compulab/cl-som-imx7/cl-som-imx7.c
index 9277094..e97efc1 100644
--- a/board/compulab/cl-som-imx7/cl-som-imx7.c
+++ b/board/compulab/cl-som-imx7/cl-som-imx7.c
@@ -11,6 +11,7 @@
 #include <env.h>
 #include <init.h>
 #include <mmc.h>
+#include <net.h>
 #include <phy.h>
 #include <netdev.h>
 #include <fsl_esdhc_imx.h>