PlatformIO: generate core_version.h when using feature/stage

[mcspr]

Original issue: platformio/platform-espressif8266#101
Doc: https://docs.platformio.org/en/latest/platforms/espressif8266.html#using-arduino-framework-with-staging-version

Update platformio-build.py:

• extract version from platform.txt (I see no other definitive source for this info)
• conditionally run makecorever.py script with the same parameters as IDE does
  (however, I am not exactly sure if VerboseAction is a right way to call it, because dependency on git dir sometimes does not work properly)

Update makecorever.py:

• fix ambiguous bytes+string error with p.stdout.readlines() data
• check if __name__ == "__main__", move generation into a function to clearly see git commands
• replace git --git-dir with just git -C, since we already know that .git dir is there
  (unless I am missing some reason for git-dir setting. if it is required, it should probably be --git-dir=.git --work-tree=. -C ...framework...)

[ivankravets]

Thanks!