Add support for Canadian cross-compiles.

Restructure the build to clean up some ambiguity (e.g., the
difference between host+target for CONFIG variables) and add
support for doing 3-way builds (BUILD != HOST != TARGET).
This is useful for building fuchsia->bare metal tools from a
linux or mac workstation.

Change-Id: I98d960f7ac4ea5bbe5df40ab9ca20c5e5aba98bd
1 file changed