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>

src/helper/options.c

@@ -113,6 +113,12 @@ static void add_default_dirs(void)
 			free(path);
 		}
 	}
+
+	path = getenv("OPENOCD_SCRIPTS");
+
+	if (path)
+		add_script_search_dir(path);
+
 #ifdef _WIN32
 	const char *appdata = getenv("APPDATA");