Porters Handbook: Update scons subsection. CH 6

[ak]

Summary

Update Scons subsection: document conversion to USES=scons, remove descriptions of variables that were removed in the conversion.

[mat]

It might be a better idea to fold all this in the USES=scons section in the USES chapter.

[ak]

In D12697#263905, @mat wrote:

It might be a better idea to fold all this in the USES=scons section in the USES chapter.

Not sure. The chapter is called 'Special Considerations' and scons really needs some. Practically every SConstruct needs to be patched to respect environment variables.