rtos: Support for "none" rtos

rtos: Support for "none" rtos

After a certain RTOS has been configured there is no mechanism
to go back to no RTOS support. It may be useful for debugging
purposes. With the provided modification, the "none" option
of RTOS is provided as a valid option.

It has been tested in two different board (Cortex M4 and Cortex M33).
Documentation has also been updated.

Signed-off-by: Asier Llano <allano@hubbell.com>
Change-Id: I602210bff31ccadd41c41e9454c52b5fffa1671e
Reviewed-on: https://review.openocd.org/c/openocd/+/7092
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>