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/freescale/ls1021atsn/ls1021atsn.c b/board/freescale/ls1021atsn/ls1021atsn.c
index 3876910..0f94099 100644
--- a/board/freescale/ls1021atsn/ls1021atsn.c
+++ b/board/freescale/ls1021atsn/ls1021atsn.c
@@ -5,6 +5,7 @@
 #include <clock_legacy.h>
 #include <fdt_support.h>
 #include <init.h>
+#include <net.h>
 #include <asm/arch-ls102xa/ls102xa_soc.h>
 #include <asm/arch/ls102xa_devdis.h>
 #include <asm/arch/immap_ls102xa.h>