Add another scripts search path for Windows builds

Add single "scripts" folder to search path for Windows OpenOCD builds
that don't use cygwin
bin/openocd.exe
scripts/interface/dummy.cfg
scripts/target/at91eb40a.cfg

Do some refactoring of current code (thx to Andreas).

Change-Id: Idbb08d1368b06f25da44f4f9ab1511db992b1724
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/785
Tested-by: jenkins