ARC: Fix configuration scripts for Windows

Paths to directories where OpenOCD will search for scripts are hardcoded at
buildtime and they absolute. That means they will not work with
cross-compiling for Windows. Paths can be setted manually by user, however
user interaction is undesirable, thus in each ARC target script relative
path to script search directory should be set, so that sourcing Digilent
script will work properly.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>