Embed API URL and firmware version from CI

[redmushie]

Hi,

I've added two environment variables to the CI:

• SHOCKLINK_API_URL that contains the API URL (api.shocklink.net by default)
• SHOCKLINK_FW_VERSION that contains:
  • On the master branch the value master-${{ github.sha }}
  • On a tag release the name of the tag, e.g. v0.7.5.

In platformio.ini I've added an extra_scripts = pre:embed.version.py to append a single build script. See the build script in question for more info.