fix(build_library): Update amd64-usr's portage profile

The usr profile is just an alias to generic now so drop the old name.
fix(build_library): Remove reference to old SDK profile

The SDK's profile changed to coreos:coreos/amd64/sdk quite a long time
ago. Remove this last reference to it so the alias can be forgotten.
feat(setup_board): Generate a complete make.conf

Add settings that are currently provided by make.conf.common-target in
coreos-overlay. Most of these are better off being derived directly from
the host environment's portage configuration.

The PKGDIR, TMPDIR, and LOGDIR values are changing to be more consistant
with the SDK configuration but these changes won't take effect until the
old make.conf.common-target is removed since its values override these.
fix(*): Use binary package for baselayout

It is no longer necessary to use the build USE flag or avoid binary
packages when installing baselayout for the first time.
fix(setup_board): Migrate packages in existing board roots.
Merge pull request #271 from marineam/profiles

Profile and config updates