bootstrap: Fix abandoned file removal.

This change makes removeAbandonedFiles perform some of the string substitutions
that are done by the bootstrap script before considering whether a built target
still exists.  It fixes a bug where the bootstrap manifest would get deleted
and subsequently regenerated, resulting in more re-bootstrap iterations than
was necessary.

Change-Id: I946c883dcc31fb77496f64d2573b395ad2cf095d