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/Makefile.am

@@ -58,6 +58,7 @@ libjtag_la_SOURCES = \
 	interface.c \
 	interfaces.c \
 	tcl.c \
+	transport.c \
 	$(DRIVERFILES)
 
 noinst_HEADERS = \
@@ -67,6 +68,7 @@ noinst_HEADERS = \
 	interfaces.h \
 	minidriver.h \
 	jtag.h \
+	transport.h \
 	minidriver/minidriver_imp.h \
 	minidummy/jtag_minidriver.h