fix(common.sh): New variables for .cache and .repo/manfests

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
fix(common.sh): Move DIGESTS handling code to common functions.

This code would be useful to use when downloading SDK tarballs in
addition to uploading them. :)
add(sdk_util.sh): Port basic SDK download logic to a shell library.

The current logic for downloading SDK tarballs is in cros_sdk and
written in python which isn't super convenient for re-using in the rest
of our shell scripts. This is a start of rewriting that logic into a
re-usable library but does not yet replace the functionality in cros_sdk.
fix(catalyst): Always use the current SDK tarball as seed.

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.
Merge pull request #135 from marineam/tarball

SDK tarball library