rtos: fix rtos no-auto configuration

rtos: fix rtos no-auto configuration

A previous fix avoiding multiple calls to the
rtos_create function had a side effect if rtos
support was configured explicitly. It affected
all rtos' that rely on symbol resolution from
gdb.

Change-Id: Id7f17c6ec5ce2450322d2748a4b2369aaa524a7b
Fixes: 3aa8bd2d1704118ad9bbee2f21cbc15a848c8c5b
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4438
Tested-by: jenkins
Reviewed-by: Richard Braun <rbraun@sceen.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>