Right now there is some funky logic to either use a previous build as a
seed or the current SDK tarball if it happens to have been downloaded.
This is a bit confusing and doesn't work reliably since it is reasonable
for there to be neither a previous build or the current SDK available if
the SDK chroot was created some time ago. Fix this by using the new SDK
library and always use the latest SDK, downloading it if needed.
A number of places refer to these paths and that number is going to
grow. Since the standard pattern is to use environment variables for
commonly used paths it is time to add ones for these:
REPO_CACHE_DIR
REPO_MANIFESTS_DIR
Use 2*CPUs for the target load average but add load average throttling
to emerge in addition to make. Also work around how catalyst sets
FEATURES so we can disable extra locking for hopefully faster builds.
