Commit e98fb7a8d3 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