Add the ability to download and unpack prebuilts

[smklein]

• Adds omicron to config/projects.toml
• Defines Prebuilt objects, which can be downloaded during gmake setup
  • Adds a caching mechanism to avoid re-downloading if the SHA-2 already matches
  • Adds an unpacking mechanism to optionally unpack the prebuilt to a well-known directory
    • Why optional? We may want to download other zone file tarballs later, without actually unpacking them
• For Omicron, downloads and unpacks global zone files, and makes them available to downstream image building via the -P flag.

[smklein]

@jclulow - Even with your changes, I still think this PR is necessary to correctly unpack the prebuilt software into a proto area.

[jclulow]

@jclulow - Even with your changes, I still think this PR is necessary to correctly unpack the prebuilt software into a proto area.

Yes I agree, but I wanted to make sure we had the requisite facilities functional for producing the image before tackling the automatic download/unpack.