adaptations for Linux

[sebastien-riou]

This pull request fixes 2 incompatibilities:

• Typically on Linux python is python2.7 and python3 is python3.x. That's a 1st source of incompatibility.
• Using platfromio.exe is the 2nd source of incompatibility since the .exe extension is meaningful only on Windows.

The build.sh takes into account 2 environment variables:

• PYTHON: default is python as in the original version
• PLATFORMIO: default is platformio.exe as in the original version

The default values ensure that the behavior is unchanged on Windows (as long as those environment variables are not defined).

The readme gives the typical values needed on Linux.

[sebastien-riou]

replaced by #5 which solves a lot more issues