No make rules for re-creating pre-built 'policy.wasm'

[berrange]

The recently released prebuilt dcap 1.22 archive contains a new file ./prebuilt/opa_bin/policy.wasm

There do not appear to be any makefile rules that generate this binary file. Given the name I'm guessing that it was probably created from the ./QuoteVerification/appraisal/qal/opa_bin/qal_script.rego file, but I'm unclear what exact opa command line to use to generate the WASM file from this rego file in a reproducible way.

Please at least document this, but preferably provide makefile rules to allow users to generate it during the build process.