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>

examples/standalone/smc911x_eeprom.c

@@ -17,6 +17,7 @@
 #include <common.h>
 #include <console.h>
 #include <exports.h>
+#include <net.h>
 #include <linux/ctype.h>
 #include <linux/types.h>
 #include "../drivers/net/smc911x.h"