tcl: introduce using_(jtag|swd|hla) helpers and use them in reset handler

Barely tested with plain SWD transport.

Change-Id: I48b59136bf4294ffed737dba01f1b30ef83aa86b
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2003
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

tcl/target/stm32w108xx.cfg

@@ -38,7 +38,7 @@ if { [info exists ENDIAN] } {
 
 swj_newdap $_CHIPNAME cpu -irlen 4 -ircapture 0x1 -irmask 0xf -expected-id $_CPUTAPID
 
-if {$using_jtag} {
+if {[using_jtag]} {
  if { [info exists BSTAPID] } {
    set _BSTAPID $BSTAPID
    jtag newtap $_CHIPNAME bs -irlen 4 -ircapture 0xe -irmask 0xf -expected-id 0x269a862b