cortex_m: Select an AP when accessing the DAP

cortex_m: Select an AP when accessing the DAP

Prepare to support multiple cortex-m cores on one DAP. Uses mem_ap_sel_*
functions and removes mem_ap_* functions. Adds a new debug_ap
parameter to the cortex_m (currently set to zero as in existing code).

Change-Id: I6926029d1e7bf44a42d453d1aff349bda824ba72
Signed-off-by: Patrick Stewart <patstew@gmail.com>
Reviewed-on: http://openocd.zylin.com/2983
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>