esp8266: Extend system microsecond counter to 64-bits; use in ticks_ms.

So now ticks_ms can count up to the full 30 bits.  Fixes issue #2412.

esp8266/esp_mphal.c

@@ -117,7 +117,7 @@ void mp_hal_debug_tx_strn_cooked(void *env, const char *str, uint32_t len) {
 }
 
 uint32_t mp_hal_ticks_ms(void) {
-    return system_get_time() / 1000;
+    return ((uint64_t)system_time_high_word << 32 | (uint64_t)system_get_time()) / 1000;
 }
 
 uint32_t mp_hal_ticks_us(void) {

esp8266/ets_alt_task.c

@@ -110,10 +110,22 @@ bool ets_post(uint8 prio, os_signal_t sig, os_param_t param) {
 
 int ets_loop_iter_disable = 0;
 
+// to implement a 64-bit wide microsecond counter
+static uint32_t system_time_prev = 0;
+uint32_t system_time_high_word = 0;
+
 bool ets_loop_iter(void) {
     if (ets_loop_iter_disable) {
         return false;
     }
+
+    // handle overflow of system microsecond counter
+    uint32_t system_time_cur = system_get_time();
+    if (system_time_cur < system_time_prev) {
+        system_time_high_word += 1; // record overflow of low 32-bits
+    }
+    system_time_prev = system_time_cur;
+
     //static unsigned cnt;
     bool progress = false;
     for (volatile struct task_entry *t = emu_tasks; t < &emu_tasks[MP_ARRAY_SIZE(emu_tasks)]; t++) {

esp8266/ets_alt_task.h

@@ -1,2 +1,4 @@
 extern int ets_loop_iter_disable;
+extern uint32_t system_time_high_word;
+
 bool ets_loop_iter(void);