improvement: make the txt file keys available to eclipse environment variables

[jantje]

Currently the variables in boards.txt platform.txt and programmers.txt are availble in project properties->C/C++ build->environment.

These environment variables are not known in higher level variable options. Like external tools or debug configurations

This issue is to make the Arduino environment variables available for the external commands

Note that Sloeber will have to guess the project and will use the active config