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

Commit e98fb7a8d3a460891f6c21fafbe62a86b50d3d1d 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
1 file changed