Fix asser_func warning/error

* Add while(1) to assert_func to avoid func returns warning
* Define a weak attr in mpconfig.h

py/mpconfig.h

@@ -381,3 +381,8 @@ typedef double mp_float_t;
 #ifndef NORETURN
 #define NORETURN __attribute__((noreturn))
 #endif
+
+// Modifier for weak functions
+#ifndef MP_WEAK
+#define MP_WEAK __attribute__((weak))
+#endif

stmhal/main.c

@@ -111,11 +111,11 @@ void nlr_jump_fail(void *val) {
 }
 
 #ifndef NDEBUG
-void __attribute__((weak))
-    __assert_func(const char *file, int line, const char *func, const char *expr) {
+void MP_WEAK __assert_func(const char *file, int line, const char *func, const char *expr) {
     (void)func;
     printf("Assertion '%s' failed, at file %s:%d\n", expr, file, line);
     __fatal_error("");
+    while (1);
 }
 #endif