PlatformIO: silence postmortem stackdump for boards using serial

[mcspr]

Dummify hardcoded ets_printf / ets_printf_P from postmortem.
It uses serial to print stackdump whenever board reboots after an error, and we do not want that when using serial connection with some external device chip.

ref: https://github.com/esp8266/Arduino/blob/f139519061bdb81dfc6d41e9cd704fd455f1647a/cores/esp8266/core_esp8266_postmortem.cpp#L85
SuperHouse/esp-open-rtos#188

[mcspr]

tested locally with espurna-core bin and Serial.begin(115200) in setup(). erase.config in telnet session prints small stack-trace to the serial in normal builds, and no longer does with this one

edit
We can't call Serial.end(), because our custom crash callback is called after serial printing is done.

Another option hinted via this linker flag:
https://github.com/esp8266/Arduino/blob/a3ea816e65029f0d79bdc19d84971f5de3858b01/tools/platformio-build.py#L88
system_restart_local is split into two functions: __wrap_system_restart_local (custom Arduino implementation) and __real_system_restart_local (original sdk function). But our callback function will not be called, so we will need to reimplement part of it and again replace it using objcopy.

Just removing printf seems easier to understand later.