rtos: add support for ChibiOS/RT 3.0

rtos: add support for ChibiOS/RT 3.0

In ChibiOS/RT 3.0 the ready list pointer "rlist" is now part of the system
data structure. Since the ready list is the first element in that
structure it can be accessed via the structure's symbol "ch".

Change-Id: Idc7eaa87cb7bbad0afa0ff1dafd54283bf429766
Signed-off-by: Christian Gudrian <christian.gudrian@gmx.de>
Reviewed-on: http://openocd.zylin.com/2352
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>