cpu/cortexm_common: allow to overwrite nmi_handler

If a user wants to handle NMI events, let them do so by
providing an implementation for nmi_handler() instead of
paniking.

cpu/cortexm_common/include/vectors_cortexm.h

@@ -80,7 +80,7 @@ void reset_handler_default(void);
  * and can not be masked (surprise surprise...). They can be triggered by
  * software and some peripherals. So far, they are not used in RIOT.
  */
-void nmi_default(void);
+void nmi_handler(void);
 
 /**
  * @brief   Hard fault exception handler

cpu/cortexm_common/vectors_cortexm.c

@@ -209,7 +209,8 @@ void reset_handler_default(void)
     kernel_init();
 }
 
-void nmi_default(void)
+__attribute__((weak))
+void nmi_handler(void)
 {
     core_panic(PANIC_NMI_HANDLER, "NMI HANDLER");
 }
@@ -505,7 +506,7 @@ ISR_VECTOR(0) const cortexm_base_t cortex_vector_base = {
         /* entry point of the program */
         [ 0] = reset_handler_default,
         /* [-14] non maskable interrupt handler */
-        [ 1] = nmi_default,
+        [ 1] = nmi_handler,
         /* [-13] hard fault exception */
         [ 2] = hard_fault_default,
         /* [-5] SW interrupt, in RIOT used for triggering context switches */