hla: add a way to pass arbitrary commands from user to layout and use for ICDI

hla: add a way to pass arbitrary commands from user to layout and use for ICDI

TI's ICDI adapter supports some additional commands which a user might
want to run for debugging or other purposes, the most useful of them
being "debug unlock" that fully mass-erases the device and unprotects
the flash.

Change-Id: I26990e736094367f92106fa891e9bb8fb0382efb
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2263
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>