Store git version of the core in the compiled binary

[igrr]

• Adds ESP.getCoreVersion()

  • for releases, returns a string with release name (e.g. "2_3_0_RC1")
  • for git version, returns a short commit hash (e.g. "dd81336b")
  • git version workflow works only on Mac and Linux (needs bash and git)
  • if the core doesn't contain .git repository (i.e. was obtained by downloading a .zip file), returns "ffffffff"
  • on Windows returns "00000000" instead of git version

• Adds ARDUINO_ESP8266_RELEASE_XYZ define which can be used in #ifdef... statements (i.e. ARDUINO_ESP8266_RELEASE_2_3_0_RC1)

• Core git version is stored at known offset in binary, so eboot will also report git version during startup (next to the last line):

  ets Jan  8 2013,rst cause:2, boot mode:(3,6)
  
  load 0x4010f000, len 1384, room 16 
  tail 8
  chksum 0x2d
  csum 0x2d
  vdd81336b
  ~ld
  

  Same caveats apply for this, i.e. it prints "v00000000" on Windows and prints "vffffffff" if there is no .git folder in the core directory.

• Relies on platform.txt so doesn't work with Platoform.IO. May or may not work with Eclipse Arduino plugin (not tested). Should work work makeEspArduino (not tested).

[codecov-io]

Current coverage is 42.60%

Merging #2099 into master will increase coverage by 16.29%

@@             master      #2099   diff @@
==========================================
  Files            19         19          
  Lines          3634       3634          
  Methods         328        328          
  Messages          0          0          
  Branches        585        585          
==========================================
+ Hits            956       1548   +592   
+ Misses         2513       2086   -427   
+ Partials        165          0   -165   

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