tools: Enable setting IGVM SVN

[ms-mahuber]

Merge Checklist

• Followed patch format from upstream recommendation: https://github.com/kata-containers/community/blob/main/CONTRIBUTING.md#patch-format
  • Included a single commit in a given PR - at least unless there are related commits and each makes sense as a change on its own.
• Aware about the PR to be merged using "create a merge commit" rather than "squash and merge" (or similar)
• genPolicy only: Ensured the tool still builds on Windows
• The upstream/missing label (or upstream/not-needed) has been set on the PR.

Summary

Allow setting SVN parameter for IGVM build scripting

Test Methodology

Ran IGVM_SVN=10 make all-confpods. This yields the following:

...
/home/azureuser/kata-containers/tools/osbuilder/igvm-builder/igvm_builder.sh -o /home/azureuser/kata-containers/tools/osbuilder -s 10
IGVM builder script
-- OUT_DIR -> /home/azureuser/kata-containers/tools/osbuilder
-- SVN -> 10
-- DISTRO -> azure-linux
-- MODE -> build
igvm_lib.sh file found. Loading content
Reading Kata image dm_verity root hash information from root_hash file
igvm_lib.sh file found. Loading content
Building (debug) IGVM files and creating their reference measurement files
{'x-ms-sevsnpvm-guestsvn': '10', 'x-ms-sevsnpvm-launchmeasurement': '2785583f67d6a72a4e3f4ac3945ca6ee10bf59d2b12d865e5e16f1dc2b4e9db5a99e22eeb6f2fd8277fc6e4747369557'}
{'x-ms-sevsnpvm-guestsvn': '10', 'x-ms-sevsnpvm-launchmeasurement': '7dd8dfccfc094dd3311ab286346f96fcb2176e7212be70bf3a5104855adc9a578e6eb6507dccaa5ceff45514e0062d54'}

...