esp8266: Let RTC work correctly after deepsleep.

By design, at wake up from deepsleep, the RTC timer will be reset, but
the data stored in RTC memory will not [1]. Therefore, we have to adjust
delta in RTC memory before going into deepsleep to get almost correct
time after waking up.

[1] http://bbs.espressif.com/viewtopic.php?t=1184#p4082

esp8266/modesp.c

@@ -46,6 +46,7 @@
 #include "espneopixel.h"
 #include "espapa102.h"
 #include "modpyb.h"
+#include "modpybrtc.h"
 
 #define MODESP_ESPCONN (0)
 
@@ -540,8 +541,11 @@ STATIC mp_obj_t esp_sleep_type(mp_uint_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(esp_sleep_type_obj, 0, 1, esp_sleep_type);
 
 STATIC mp_obj_t esp_deepsleep(mp_uint_t n_args, const mp_obj_t *args) {
+    uint32_t sleep_us = n_args > 0 ? mp_obj_get_int(args[0]) : 0;
+    // prepare for RTC reset at wake up
+    rtc_prepare_deepsleep(sleep_us);
     system_deep_sleep_set_option(n_args > 1 ? mp_obj_get_int(args[1]) : 0);
-    system_deep_sleep(n_args > 0 ? mp_obj_get_int(args[0]) : 0);
+    system_deep_sleep(sleep_us);
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(esp_deepsleep_obj, 0, 2, esp_deepsleep);

esp8266/modmachine.c

@@ -103,6 +103,8 @@ STATIC mp_obj_t machine_deepsleep(void) {
         }
     }
 
+    // prepare for RTC reset at wake up
+    rtc_prepare_deepsleep(sleep_us);
     // put the device in a deep-sleep state
     system_deep_sleep_set_option(0); // default power down mode; TODO check this
     system_deep_sleep(sleep_us);

esp8266/modpybrtc.c

@@ -120,6 +120,12 @@ uint64_t pyb_rtc_get_us_since_2000() {
     return (((uint64_t)rtc_ticks * cal) >> 12) + delta;
 };
 
+void rtc_prepare_deepsleep(uint64_t sleep_us) {
+    // RTC time will reset at wake up. Let's be preared for this.
+    int64_t delta = pyb_rtc_get_us_since_2000() + sleep_us;
+    system_rtc_mem_write(MEM_DELTA_ADDR, &delta, sizeof(delta));
+}
+
 STATIC mp_obj_t pyb_rtc_datetime(mp_uint_t n_args, const mp_obj_t *args) {
     if (n_args == 1) {
         // Get time

esp8266/modpybrtc.h

@@ -30,3 +30,5 @@ extern uint64_t pyb_rtc_alarm0_expiry;
 void pyb_rtc_set_us_since_2000(uint64_t nowus);
 
 uint64_t pyb_rtc_get_us_since_2000();
+
+void rtc_prepare_deepsleep(uint64_t sleep_us);