feat(esp8266): Add rtc related attribute
// Use as ets_printf(DRAM_STR("Hello world!\n"));
#define DRAM_STR(str) (__extension__({static const DRAM_ATTR char __c[] = (str); (const char *)&__c;}))
+// Forces data into RTC memory.
+// Any variable marked with this attribute will keep its value
+// during a deep sleep / wake cycle.
+#define RTC_DATA_ATTR __attribute__((section(".rtc.data")))
+
+// Forces read-only data into RTC memory.
+#define RTC_RODATA_ATTR __attribute__((section(".rtc.rodata")))
+
#endif /* __ESP_ATTR_H__ */
