[ns8390] Use stub files instead of src/Config

ns8390.c can produce four different drivers (one PCI, three ISA.)  The
ISA driver requires setting a few macros; do that by setting defines
in stub files instead of using src/Config.

Currently, all the ISA drivers are broken (they were not enabled by
default), so #if 0 them out.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

src/drivers/net/3c503.c

@@ -0,0 +1,5 @@
+/* 3Com 3c503, a memory-mapped NS8390-based card */
+#if 0 /* Currently broken! */
+#define INCLUDE_3C503
+#include "ns8390.c"
+#endif