The remaining 4 kB had been reserved for storing RF calibration and
BT stack state since 4e092be6. However, these features never got
implemented. If we ever need to place RF related data into RTC slow
memory, we can do this by creating a variable with RTC_NOINIT_ATTR
instead.
Closes https://github.com/espressif/esp-idf/issues/3993
1. separate rom include files and linkscript to esp_rom
2. modefiy "include rom/xxx.h" to "include esp32/rom/xxx.h"
3. Forward compatible
4. update mqtt