unify scb (system control block)

these register definitions are common to all cortex mcus. some of the
registers might not be implemented everywhere (especially the floating
point registers), but defining them does no harm.

this modification does not result in any changes in the example
binaries.

lib/stm32/f4/vector_chipset.c

@@ -18,7 +18,7 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include <libopencm3/stm32/f4/scb.h>
+#include <libopencm3/cm3/scb.h>
 
 static void pre_main(void)
 {