cortex-m4_common: Add debugger break in crash.c

cpu/cortex-m4_common/crash.c

@@ -52,6 +52,8 @@ NORETURN void core_panic(int crash_code, const char *message)
     /* disable watchdog and all possible sources of interrupts */
     disableIRQ();
 #if DEVELHELP
+    /* The bkpt instruction will signal to the debugger to break here. */
+    __ASM("bkpt #0");
     /* enter infinite loop, into deepest possible sleep mode */
     while (1) {
         lpm_set(LPM_OFF);