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>

doc/openocd.texi

@@ -700,6 +700,7 @@ Configuration files and scripts are searched for in
 @item any search dir specified on the command line using the @option{-s} option,
 @item any search dir specified using the @command{add_script_search_dir} command,
 @item @file{$HOME/.openocd} (not on Windows),
+@item a directory in the @env{OPENOCD_SCRIPTS} environment variable (if set),
 @item the site wide script library @file{$pkgdatadir/site} and
 @item the OpenOCD-supplied script library @file{$pkgdatadir/scripts}.
 @end enumerate

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");