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>

cmd/ethsw.c

@@ -11,6 +11,7 @@
 #include <errno.h>
 #include <env_flags.h>
 #include <ethsw.h>
+#include <net.h>
 
 static const char *ethsw_name;