The following commit updates the first half of the the system
example README files. Some other changes were also made:
* Updated base_mac_address example
* Moved contents in GCOV README to GCOV docs
* Some *main.c file names updated
* Updated example README template
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
