mbed_die: Use wait_us not wait_ms

mbed_die was calling wait_ms in a critical section - this is deprecated
behaviour, and caused a fatal error in debug builds, potentially leading
to an infinite reboot loop if this was caused due to a reboot limit
halt.

Switch to using wait_us - this is safe in a critical section. This does
trigger a call to mbed_warning, due to the large parameter, but that is
harmless - it doesn't output anything to the console, and won't
overwrite the error context if it already contains something.

Author: kjbracey

platform/mbed_board.c

@@ -33,16 +33,16 @@ WEAK MBED_NORETURN void mbed_die(void)
     while (1) {
         for (int i = 0; i < 4; ++i) {
             gpio_write(&led_err, 1);
-            wait_ms(150);
+            wait_us(150000);
             gpio_write(&led_err, 0);
-            wait_ms(150);
+            wait_us(150000);
         }
 
         for (int i = 0; i < 4; ++i) {
             gpio_write(&led_err, 1);
-            wait_ms(400);
+            wait_us(400000);
             gpio_write(&led_err, 0);
-            wait_ms(400);
+            wait_us(400000);
         }
     }
 }