Clean up multi-arch support

[bgilbert]

We currently wrap the architecture name into a board for historical reasons, and sometimes we treat it as a per-platform option. Cleanups are needed.

[bgilbert]

See coreos/mantle#940 (comment).

[arithx]

Here's some goals I'd like to see as an outcome of this refactor:

• Change current terminology from CL specific board to architecture; update code that requires board to fetch the CL version of a given architecture
• Utilize distribution flag when gating architectures during run-time
• Add flag to build script to either allow a specified list of architectures to build (defaulting to all supported) or specify the distribution mantle is being built for (so that CL won't try to build architectures it doesn't/can't support)
• Remove architecture from kola/ore platform-specific options in favor of a generic option
• Store the architecture in the Flight object, change kola runner references to architecture to fetch from the Flight

[dustymabe]

We've had some cleanups around this but don't have anything specific planned further. We'll close this and open PRs as new ideas come up.