cortex_m: allow setting debug ap during create

This patch adds a Cortex-M private configuration option
that allows setting the acess point during target
creation. This circumvents situations in hybrid systems
when the correct access point can not be automatically
detected.

Change-Id: If313a5250e6e66509bb9080f3498feab7781dced
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3639


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>