releasetools: Allow using futility-host to sign old TF.zip.

Commit e98fb7a8 switched to using
futility-host instead of the prebuilt futility. This CL adds support to
handle signing old TF.zip that still says "futility=prebuilt/..." in
META/misc_info.txt.

Bug: 35467608
Test: Generate otatools.zip and sign an old ryu TF.zip.
Change-Id: I48a9cc918c7afce361e1ec9bc4f85f74fa92566e
(cherry picked from commit 4f104d1b)