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>

src/target/cortex_m.h

@@ -188,6 +188,8 @@ struct cortex_m_common {
 	enum cortex_m_isrmasking_mode isrmasking_mode;
 
 	struct armv7m_common armv7m;
+
+	int apsel;
 };
 
 static inline struct cortex_m_common *