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/stm32f4x.cfg

@@ -47,7 +47,7 @@ if { [info exists BSTAPID] } {
   set _BSTAPID2 0x06419041
 }
 
-if {$using_jtag} {
+if {[using_jtag]} {
  jtag newtap $_CHIPNAME bs -irlen 5 -expected-id $_BSTAPID1 \
 	-expected-id $_BSTAPID2
 }
@@ -69,7 +69,7 @@ flash bank $_FLASHNAME stm32f2x 0 0 0 0 $_TARGETNAME
 adapter_khz 1000
 
 adapter_nsrst_delay 100
-if {$using_jtag} {
+if {[using_jtag]} {
  jtag_ntrst_delay 100
 }