Runtime configs have been implemented for awhile, but disabled. This patch
enables configuration. Config files will be loaded in the following order
(later files override earlier ones):
1a. $MACHRC
1b. $MOZBUILD_STATE_PATH/machrc (if $MACHRC is unset)
2. topsrcdir/machrc
3. CLI via --settings
Note: .machrc may be used instead of machrc if desired.
MozReview-Commit-ID: IntONAZLGML