openocd: prepare for jimtcl 0.81 'expr' syntax change

openocd: 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'.
All the scripts distributed with OpenOCD are already compliant
with the new syntax.

To avoid breaking user script, introduce a replacement for 'expr'
command that handles the old syntax while issuing a deprecated
warning.
This change should be part of OpenOCD v0.12.0, then reverted.

Change-Id: Ib08aa8ebcb634c81a3ce9d24fb4938b0418c947c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6510
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Tested-by: jenkins