startup.tcl: prepare for jimtcl 0.81 'expr' syntax change

startup.tcl: prepare for jimtcl 0.81 'expr' syntax change

Jimtcl commit 1843b79a03dd ("expr: TIP 526, only support a single
arg") drops the support for multi-argument syntax for the TCL
command 'expr'.

Modify the script startup.tcl compiled-in OpenOCD binary to comply
with the new jimtcl.

Change-Id: I520dcafacadaa289a815035f93f250447ca66ea0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6158
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>