Allow use of Environment Variable in Configuration File

[mdhorn]

It would be very advantageous to allow the use of environment variables, such a $HOME, in the builder configuration file. When used in an automated build environment like Jenkins or Travis, it would be helpful to use environment variables for the various paths.

NOTE: This will require a compatible change to the mixer tool as it also reads the same configuration file.

See clearlinux/mixer-tools#28

[tmarcu]

Merged, will be in official release.