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>

include/common.h

@@ -36,7 +36,6 @@
 #include <init.h>
 #include <display_options.h>
 #include <vsprintf.h>
-#include <net.h>
 #include <bootstage.h>
 #endif	/* __ASSEMBLY__ */