feat(esp8266): Add rtc segment in ld file

components/esp8266/ld/esp8266.common.ld

@@ -62,6 +62,15 @@ SECTIONS
     _dport0_data_end = ABSOLUTE(.);
   } >dport0_0_seg :dport0_0_phdr
 
+  /* RTC memory holds user's data/rodata */
+  .rtc.data :
+  {
+    _rtc_data_start = ABSOLUTE(.);
+    *(.rtc.data)
+    *(.rtc.rodata)
+    _rtc_data_end = ABSOLUTE(.);
+  } > rtc_seg
+
   .text : ALIGN(4)
   {
     _stext = .;

components/esp8266/ld/esp8266.ld

@@ -19,7 +19,16 @@
 MEMORY
 {
   dport0_0_seg :    org = 0x3FF00000, len = 0x10
+
+  /* All .data/.bss/heap are in this segment. */
   dram0_0_seg  :    org = 0x3FFE8000, len = 0x18000
+
+  /* Functions which are critical should be put in this segment. */
   iram1_0_seg  :    org = 0x40100000, len = 0x8000
-  irom0_0_seg :                         org = 0x40201010, len = 0xE0000
+
+  /* It is actually mapped to flash. */
+  irom0_0_seg  :    org = 0x40201010, len = 0xFF000 - 0x10
+
+  /* RTC memory, persists over deep sleep. */
+  rtc_seg      :    org = 0x60001200, len = 0x200
 }