WIP: Manage libboost and cmake

[PhilippvK]

This aims to reduce dependencies on system (Ubuntu) packages by (optionally) allowing to build cmake and libboost from source. The default environment should still use the versions available on the system.

Not implemented yet:

• Use custom libboost while building etiss/spike
• Use custom libboost while running spike/etiss_pulpino
• Use custom cmake for building other dependencies
• Use custom cmake in platforms
• Add mechanism to turn off building boost/cmake from source (e.g. in environment.yml via setting cmake.exe/boost.install_dir (implicitly) or cmake.from_source: false/boost.from_source: false (explicitly)