rtos: use ARRAY_SIZE() and simplify rtos_type.create()

rtos: use ARRAY_SIZE() and simplify rtos_type.create()

Use the existing macro ARRAY_SIZE().
Rewrite the functions rtos_type.create() to simplify the logic.

Change-Id: I8833354767045d1642801d26944c9087a77add00
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6261
Tested-by: jenkins