Fix BoardContext compilation break on develop

[emrainey]

Compilation of BoardContext fails on develop because:

1. BoardContext.cpp declares linker symbols using the LINKER_TYPED_SYMBOL macro, but does not include the <cortex/linker.hpp> header where it is defined.
2. BoardContext.hpp declared GetEthernet() and GlobalContext.cpp enlisted it, but the net module is not yet present on develop.
3. Demo.hpp included the non-existent jarnax/net/ethernet/Driver.hpp.

This task will:

• Include <cortex/linker.hpp> in BoardContext.cpp.
• Comment out/remove the premature ethernet references in BoardContext.hpp, GlobalContext.cpp, and Demo.hpp.

[emrainey]

PR #36 has been opened to fix the compilation break by including the missing cortex/linker.hpp header and commenting out the premature Ethernet references.