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/b4860qds/eth_b4860qds.c b/board/freescale/b4860qds/eth_b4860qds.c
index 67fb1b9b..95ec0b9 100644
--- a/board/freescale/b4860qds/eth_b4860qds.c
+++ b/board/freescale/b4860qds/eth_b4860qds.c
@@ -19,6 +19,7 @@
  */
 
 #include <common.h>
+#include <net.h>
 #include <netdev.h>
 #include <asm/fsl_serdes.h>
 #include <fm_eth.h>