transport selection tweaks

* Bugfix and simplify legacy jtag-only defaulting
* Make "dummy" declare its jtag-only nature
* likewise update ft2232
* warn if selection is _required_ (multi-transport adapters),

fixes the "only ft2232 works" bug for at least dummy, with
other drivers going the "legacy" path (submit patches).

Signed-off-by: David Brownell <db@helium.(none)>

src/jtag/interface.h

@@ -206,6 +206,9 @@ struct jtag_interface {
 	unsigned supported;
 #define DEBUG_CAP_TMS_SEQ	(1 << 0)
 
+	/** transports supported in C code (NULL terminated vector) */
+	const char **transports;
+
 	/**
 	 * Execute queued commands.
 	 * @returns ERROR_OK on success, or an error code on failure.
@@ -289,4 +292,6 @@ struct jtag_interface {
 };
 
 
+extern const char *jtag_only[];
+
 #endif // OPENOCD_JTAG_INTERFACE_H