cfg: allow stm32discovery parameter override

This enable the user or board config to override the parameters
passed to stm32_stlink.cfg.

Required to fix a incorrect working area bug with the stm32vldiscovery.

Change-Id: I40a4f7913ff37d577d44b1f23befccf0317080a1
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/597
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

tcl/target/stm32f2x_stlink.cfg

@@ -2,9 +2,17 @@
 # STM32f2x stlink pseudo target
 #
 
-set CHIPNAME stm32f2x
-set CPUTAPID 0x2ba01477
-set WORKAREASIZE 0x10000
+if { [info exists CHIPNAME] == 0 } {
+   set CHIPNAME stm32f2x
+}
+
+if { [info exists CPUTAPID] == 0 } {
+   set CPUTAPID 0x2ba01477
+}
+
+if { [info exists WORKAREASIZE] == 0 } {
+   set WORKAREASIZE 0x10000
+}
 
 source [find target/stm32_stlink.cfg]