Build FUTILITY tool from futility-host.

Also pack the tool, vboot_signer.sh script and the test keys into
otatools.zip. This allows signing ryu target without a source repo.

Bug: 35467608
Test: m otatools-package and check the newly included files.
Test: Use otatools.zip to sign a ryu target.
Change-Id: I70f7b359f21a579463d5c7251daac81a97e338cc