Evaluate environment variables recipe/tool definitions

[micahsnyder]

It is currently not possible to use environment variables in recipe/tool definitions.
For example, a user-only (not system-wide) install of NASM compiler will install to:
"%userprofile%\AppData\Local\bin\NASM\nasm.exe"

Python's os.path.expandvars() function would solve this, and should be applied anywhere than environment variable support is needed (tool check strings, build scripts, etc).