tool: add cargo xflowey custom-vmfirmwareigvm-dll

[daprilik]

To quote the tool's CLI help:

Encapsulate an existing pre-built IGVM file into unsigned
vmfirmwareigvm.dll resource DLL.

Unlike build-igvm, this tool will NOT build OpenHCL from scratch. This
tool streamlines the process of building the in-tree vmfirmwareigvm_dll
crate (which requires setting various env vars, installing certain
dependencies, etc...).

NOTE: This tool is primarily intended for use by Microsoft employees, as
open-source deployments of OpenHCL typically load the IGVM file directly
(rather than being encapsulated in a resource-DLL).

Unfortunately, I still don't quite know what magic is required to properly cross-compile the vmfirmwareigvm_dll crate from WSL2 / Linux, as the dependency on rc.exe makes things a bit more complicated than the typical windows cross-compile case. In the meantime, I've added some checks to ensure folks are only running this tool on native windows.

[daprilik]

note: this is just a drive-by cleanup

[mattkur]

Thanks for this! Would you add usage instructions to the guide?

[daprilik]

Thanks for this! Would you add usage instructions to the guide?

Given that this is a Microsoft-employee oriented tool, I'm inclined to say that the documentation in the CLI is sufficient in the context of open-source. That said, I'll send out a PR which updates our Microsoft-internal guide to include a pointer to this new tool, as well as some extra docs on how to actually use the DLL internally.