* Linker was choosing ROM symbols for these, which use integer soft-float
operations and are much slower.
* _divsf3() moved to IRAM to avoid regressions with any code that does
integer float division in IRAM interrupt handlers (+88 bytes IRAM)
* Thanks to michal for reporting:
https://esp32.com/viewtopic.php?f=14&t=10540&p=43367
Removes the need to know/guess the paths to these libraries. Once we are gcc 8 only, we
can remove -nostdlib and no additional arguments are needed for system libraries.
The catch is: any time IDF overrides a symbol in the toolchain sysroot, we need
an undefined linker marker to make sure this symbol is seen by linker.
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
