Fix "Evaluate 'script' in the global scope"

Fix "Evaluate 'script' in the global scope"

This fixes commit Evaluate 'script' in the global scope. It caused
Windows builds behave differently than before because path was evaluated twice
and backslashes from Windows' paths got unescaped and effectively wiped out.
Configs could only be passed with "-f ../dir/config.cfg" or "-f
..\\dir\\config.cfg" instead of usual "-f dir/config.cfg" (or using backslash)
as previously.

Change-Id: I13b4abac6dbe6d770cc11a4e61c9421ef340da83
Author: Steve Bennett <steveb@workware.net.au>
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/40
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>