helper/options: Use OPENOCD_SCRIPTS dir if set.

helper/options: Use OPENOCD_SCRIPTS dir if set.

This makes it easier to relocate the install tree of OpenOCD from where
it was originally built (for example, if put onto a different machine),
without having to change scripts or add something to the command line
every time.

Change-Id: Ia5edf0eba166f7a999f267bd6a92402dab9b399e
Signed-off-by: Jonathan Larmour <jifl@eCosCentric.com>
Reviewed-on: http://openocd.zylin.com/3004
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>