helper/command: do not replace new commands with ocd_ prefix

helper/command: do not replace new commands with ocd_ prefix

The TCL return values are now consistent, no need anymore for the
hack of registering the commands with "ocd_" prefix and override
them with proc ocd_bouncer.

Clean-up the command registration and remove the proc ocd_bouncer.

This change was part of http://openocd.zylin.com/1815 from Paul
Fertser and has been extracted and rebased to simplify the review.

Change-Id: I2a467e73ecb068686ea3fda91bf961aba6db6427
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5086
Tested-by: jenkins