Fix the .toolchains path. Add some build notes.

[treytomes]

This was required for me to successfully complete the build process in an Ubuntu system running under VirtualBox.

[samuelh2005]

For me this dosen't quite work on a Raspiberry Pi; I get the following error

scons: Reading SConscript files ...
NameError: name 'os' is not defined:
  File "/mnt/data_all/projects/gtmos/SConstruct", line 50:
    HOST_ENVIRONMENT = Environment(variables=VARS,
  File "/usr/lib/python3/dist-packages/SCons/Environment.py", line 982:
    variables.Update(self)
  File "/usr/lib/python3/dist-packages/SCons/Variables/__init__.py", line 187:
    exec(contents, {}, values)
  File "<string>", line 8:

However, the SConscript file can be read now!

[treytomes]

I don't have a Raspberry Pi to test on right now. Interesting that the os package would be missing there; I thought it would be standardized across processors.

[samuelh2005]

I'm able to import os inside a normal python program. It might be how scons do things.

[samuelh2005]

Also on an Ubuntu VM inside WSL2 the process seems fine. Probably just a bug in Raspberry Pi python.