Add Host stubs for RTC mem read/write

[mikee47]

This PR adds stubs for the system_rtc_mem_read and system_rtc_mem_write functions so that code compiles for the Host arch.

No immediate use-cases fo this, but for future consideration it might be useful to make these functional using a backing file, similar to how flash memory is handled. One area where this might be useful is improving System.restart() behaviour, perhaps by forking a separate process which can be restarted.

[what-the-diff]

• Added system_rtc_mem_read and system_rtc_mem_write functions to esp-hal.
• These are used by the ESP8266 SDK for reading/writing RTC memory, which is not supported in Sming yet (and probably never will be).
• The return value of these functions was changed from void* to bool so that they can indicate success or failure when called with invalid parameters (e.g., srcAddr > 127) without crashing the program due to a NULL pointer dereference on line 122 of rboot-ota.cpp: memset(data->buffer + data->length, 0xFF, len);