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/barco/platinum/platinum.c b/board/barco/platinum/platinum.c
index c2c7c09..7984647 100644
--- a/board/barco/platinum/platinum.c
+++ b/board/barco/platinum/platinum.c
@@ -9,6 +9,7 @@
 #include <mmc.h>
 #include <fsl_esdhc_imx.h>
 #include <miiphy.h>
+#include <net.h>
 #include <netdev.h>
 #include <asm/io.h>
 #include <asm/arch/clock.h>