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/f1/rcc.c

@@ -48,6 +48,7 @@ LGPL License Terms @ref lgpl_license
 
 /**@{*/
 
+#include <libopencm3/cm3/assert.h>
 #include <libopencm3/stm32/f1/rcc.h>
 #include <libopencm3/stm32/f1/flash.h>
 
@@ -166,8 +167,7 @@ int rcc_osc_ready_int_flag(osc_t osc)
 		break;
 	}
 
-	/* Shouldn't be reached. */
-	return -1;
+	cm3_assert_not_reached();
 }
 
 /*-----------------------------------------------------------------------------*/