Vendor the Gummiboot UEFI stub loader

[ahesford]

We need to update generate-zbm to point to the vendored location before looking for gummiboot or systemd-boot versions, but I'm thinking we should actually look in seven places for the vendored file:

./zfsbootmenu/stub
../zfsbootmenu/stub
../share/zfsbootmenu/stub
dirname(__FILE__)/zfsbootmenu/stub
dirname(__FILE__)/../zfsbootmenu/stub
dirname(__FILE__)/../share/zfsbootmenu/stub
/usr/share/zfsbootmenu/stub

That covers common cases like running generate-zbm directly from the repo, running it in some installed prefix and (maybe) running in our build container. (This needs to be tested.) I'm looking for some suggestions on this search.

Once we have the desired behavior, we should stop installing gummiboot-efistub in the build container.

[zdykstra]

Before this is vendored, I'd like to check kernel configs for Debian, Arch and maybe Gentoo to confirm that they have the right EFI handoff option enabled. If they don't, this won't get us very far.

[zdykstra]

Arch's 6.3.x kernel ships with CONFIG_EFI_HANDOVER_PROTOCOL enabled. Ubuntu 23.10's 6.5.x kernel ships with it enabled.

[ahesford]

Because generate-zbm is now more intelligent about writing EFI bundles, this is less relevant.