initial "transport" framework

This adds the guts of a transport framework with initialization,
which should work with current JTAG-only configurations (tested
with FT2232).

Each debug adapter can declare the transports it supports, and
exactly one transport is initialized.  (with its commands) in
any given OpenOCD session.

  * Define a new "struct transport with init hooks and a few
 "transport"  subcommands to support it:

     "list" ... list the transports configured (just "jtag" for now)
     "select" ... makes the debug session use that transport
     "init" ... initializes the selected transport (internal)

  * "interface_transports" ... declares transports the current interface
    can support.  (Some will do this from C code instead, when there are
    no hardware versioning (or other) issues to prevent it.

Plus some FT2232 tweaks, including a few to streamline upcoming
support for an SWD transport (initially for Luminary adapters).

Eventually src/jtag should probably become src/transport, moving
jtag-specific stuff  to transport/jtag.

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

src/jtag/adapter.c

@@ -35,6 +35,7 @@
 #include "minidriver.h"
 #include "interface.h"
 #include "interfaces.h"
+#include "transport.h"
 
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
@@ -92,6 +93,25 @@ static int default_srst_asserted(int *srst_asserted)
 	return ERROR_OK;
 }
 
+COMMAND_HANDLER(interface_transport_command)
+{
+	char **transports;
+	int retval;
+
+	retval = CALL_COMMAND_HANDLER(transport_list_parse, &transports);
+	if (retval != ERROR_OK) {
+		return retval;
+
+	retval = allow_transports(CMD_CTX, (const char **)transports);
+	if (retval != ERROR_OK) {
+		for (unsigned i = 0; transports[i]; i++)
+			free(transports[i]);
+		free(transports);
+	}
+	}
+	return retval;
+}
+
 COMMAND_HANDLER(handle_interface_list_command)
 {
 	if (strcmp(CMD_NAME, "interface_list") == 0 && CMD_ARGC > 0)
@@ -451,6 +471,13 @@ static const struct command_registration interface_command_handlers[] = {
 		.help = "Select a debug adapter interface (driver)",
 		.usage = "driver_name",
 	},
+	{
+		.name = "interface_transports",
+		.handler = interface_transport_command,
+		.mode = COMMAND_CONFIG,
+		.help = "Declare transports the interface supports.",
+		.usage = "transport ... ",
+	},
 	{
 		.name = "interface_list",
 		.handler = handle_interface_list_command,