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>

doc/openocd.texi

@@ -3100,6 +3100,11 @@ Specifies the adapter layout to use.
 The vendor ID and product ID of the device.
 @end deffn
 
+@deffn {Command} {hla_command} command
+Execute a custom adapter-specific command. The @var{command} string is
+passed as is to the underlying adapter layout handler.
+@end deffn
+
 @deffn {Config Command} {trace} source_clock_hz [output_file_path]
 Enable SWO tracing (if supported). The source clock rate for the
 trace port must be specified, this is typically the CPU clock rate. If