cortex_m: read and display core security state

Change-Id: I0fce3c66af7e98df2dc2258daf0d6af661e29ae7
Signed-off-by: Laurent LEMELE <laurent.lemele@st.com>
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5798
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/arm.h

@@ -197,6 +197,9 @@ struct arm {
 	/** Flag reporting armv6m based core. */
 	bool is_armv6m;
 
+	/** Flag reporting armv8m based core. */
+	bool is_armv8m;
+
 	/** Floating point or VFP version, 0 if disabled. */
 	int arm_vfp_version;