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/cavium/thunderx/thunderx.c b/board/cavium/thunderx/thunderx.c
index 687ade5..1b1b9d5 100644
--- a/board/cavium/thunderx/thunderx.c
+++ b/board/cavium/thunderx/thunderx.c
@@ -9,6 +9,7 @@
 #include <init.h>
 #include <malloc.h>
 #include <errno.h>
+#include <net.h>
 #include <linux/compiler.h>
 
 #include <cavium/atf.h>