Make supports specifying all types of variables on the command line
(using =, :=, +=, and other variable references. When running soong_ui
through make/makeparallel these all effectively become environment
variables.
So in preparation to remove the Make wrapper, support a simplified form
of this syntax, roughly equivalent to what the shell supports if
specified before the command (<NAME>=<VALUE>).
Test: m -j blueprint_tools
Change-Id: I08fa2b86710f282e619b0cc324a3e5bbaf62d26e
091525e15c