Update kzip script to support superproject revision environment variable

This changes the superproject sha variable to use the
SUPERPROJECT_REVISION as the default value if no explicit sha is
defined. Only sets up the new default if the revision value matches a
sha.

This change is required since the builder provides a revision for the
superproject that is either pinned to a sha or a branch name. If it is
pinned to a sha, we want to use this as the fallback source for
SUPERPROJECT_SHA

Bug: b/149248752
Change-Id: I59569b0e4d2f360243f0adc5b2587130956b6b99