This commit updates the visibility of various header files and cleans up
some unnecessary inclusions. Also, this commit removes certain header
include paths which were maintained for backward compatibility.
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