cfg: add ST-LINK TRANSPORT config override

This enables the user to override the transport used for st-link.
If JTAG is selected it will also change the default id used to the JTAG id
rather than the SW-DP id.

Change-Id: I4fe352e4932e2f4ec278168e99ba2d2d50fd850a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/443
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>

tcl/target/stm32_stlink.cfg

@@ -23,10 +23,22 @@ if { [info exists CPUTAPID] } {
    set _CPUTAPID 0x1ba01477
 }
 
+if { [info exists TRANSPORT] } {
+   set _TRANSPORT $TRANSPORT
+   if { $TRANSPORT == "stlink_jtag" } {
+      if { [info exists CPUTAPID] == 0 } {
+         # jtag requires us to use the jtag tap id
+         set _CPUTAPID 0x3ba00477
+      }
+   }
+} else {
+   set _TRANSPORT stlink_swd
+}
+
 #
 # possibles value are stlink_swd or stlink_jtag
 #
-transport select stlink_swd
+transport select $_TRANSPORT
 
 stlink newtap $_CHIPNAME cpu -expected-id $_CPUTAPID