rtos: run rtos create hook only once on autodetect

rtos: run rtos create hook only once on autodetect

Whenever a "qSymbol" from gdb is received, the rtos "create"
function was called. Make sure this happens only once and only
if rtos autodetection is used.

Change-Id: Ie5f8632cfce2d64a38dbdb63468302c4e8a784f4
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3998
Tested-by: jenkins
Reviewed-by: Sergey A. Borshch <sb-sf@users.sourceforge.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>