Drop CL support from mantle

[bgilbert]

CL is pretty deep in maintenance mode now, and mantle won't receive extensive further changes to support it. To make it easier to evolve mantle for FCOS and RHCOS, we've created a mantle cl branch to house Container Linux support. Once CL CI has been switched over to the new branch, we can drop CL support from the master branch. That includes:

• [x] Drop cork and gangue
• [ ] Drop SDK support code
• [ ] Change default paths in arguments to not point into the SDK
• [ ] Switch board to architecture and drop -usr extension
• [x] Drop CL-specific kola tests
• [ ] Drop CL-specific hacks

[arithx]

Switch board to architecture and drop -usr extension is split out as coreos/coreos-assembler#1170

[nikita-dubrovskii]

Done