tools: setup python3 directories successfully
make: Update message if old envvariables are found

The new warning is more direct about what to do to fix the warning.
make docs: Better intro to make system readme

The make system readme now gives a basic primer on how to compile tinyos
applications.
tools: do not compile msp430/cppbsl on mac

This tools explicitly looks for `linux/serial.h` which will not exist on
a mac.
Clarify how the new approach works.
tools: copy jar files into $prefix/lib/java

Only if requested by ./configure --prefix=...