helper/command: add macro CMD

helper/command: add macro CMD

Accessing the variable "cmd" in a COMMAND_HANDLER is confusing
because the variable definition is not visible in the prototype,
thus "cmd" can be identified by the reader as a global variable
defined somewhere else.

Add the new macro CMD to make more evident it is a part of the
COMMAND_HANDLER.

Change-Id: I8f63da593a931045cc404323f143b88c5c80ab5e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5047
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>