Make mg_random() fallible. Abort TLS on weak RNG

examples/wch/ch32v307-make-baremetal-builtin/hal.c

@@ -26,11 +26,12 @@ __attribute__((interrupt())) void SysTick_Handler(void) {
   SysTick->SR = 0;
 }
 
-void mg_random(void *buf, size_t len) {  // Use on-board RNG
+bool mg_random(void *buf, size_t len) {  // Use on-board RNG
   for (size_t n = 0; n < len; n += sizeof(uint32_t)) {
     uint32_t r = rng_read();
     memcpy((char *) buf + n, &r, n + sizeof(r) > len ? len - n : sizeof(r));
   }
+  return true;
 }
 
 uint64_t mg_millis(void) {  // Let Mongoose use our uptime function