Add assert macros to aid in debugging.

Adds libopencm3/cm3/assert.h header that provides assertion check macros
similar to those provided by the standard C library.

Thanks to Nicolas Schodet for help.

lib/stm32/f2/rcc.c

@@ -19,6 +19,7 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <libopencm3/cm3/assert.h>
 #include <libopencm3/stm32/f2/rcc.h>
 #include <libopencm3/stm32/f2/flash.h>
 
@@ -125,8 +126,7 @@ int rcc_osc_ready_int_flag(osc_t osc)
 		break;
 	}
 
-	/* Shouldn't be reached. */
-	return -1;
+	cm3_assert_not_reached();
 }
 
 void rcc_css_int_clear(void)