efibuild: Add support for DISCARD_SUBMODULES

[mikebeaton]

This is faster to build than using DISCARD_PACKAGES since OpenCorePkg does not need to be cloned (which is relatively slow) then deleted. To implement in OpenCore requires changing build_duet.tool and build_oc.tool to set and export DISCARD_SUBMODULES instead of DISCARD_PACKAGES. (Not sure if there are other projects that can be updated too, though DISCARD_PACKAGES is still supported.)

Requires new-enough git to work, though I believe that is not an issue here - unless we perhaps need to support building on user-owned machines too old to have this?

[mikebeaton]

Have added this test commit https://github.com/mikebeaton/OpenCorePkg/commits/discard-test which directly uses efibuild.sh from this PR, and confirms that the full CI build works using this change.

[savvamitrofanov]

This is faster to build than using DISCARD_PACKAGES since OpenCorePkg does not need to be cloned (which is relatively slow) then deleted. To implement in OpenCore requires changing build_duet.tool and build_oc.tool to set and export DISCARD_SUBMODULES instead of DISCARD_PACKAGES. (Not sure if there are other projects that can be updated too, though DISCARD_PACKAGES is still supported.)

Requires new-enough git to work, though I believe that is not an issue here - unless we perhaps need to support building on user-owned machines too old to have this?

LGTM, but I suggest to remove submodule in such way:

git submodule deinit <submodule-name>    
git rm <submodule-name>

It should be compatible across different git versions

[mikebeaton]

I'm not sure if the git submodule deinit call is needed, because this code is called only when the submodules have not yet been initialised (at least it is now, with a submodules.ready guard file, which I have also added; so this discard code should only happen when the repo is being first checked out, before the first call to git submodule update --init).

So in the current version (git submodule deinit included) we get the following log messages:

error: could not lock config file .git/modules/OpenCorePkg/config: No such file or directory
warning: Could not unset core.worktree setting in submodule 'OpenCorePkg'
rm 'OpenCorePkg'
[master c0718d6] Discarded submodules

[mikebeaton]

I've added a third commit, removing git submodule deinit again, but keeping the guard file so that this is definitely all only done once (before init). I think this is right. (Full build retested at https://github.com/mikebeaton/OpenCorePkg/tree/discard-test.)

[mikebeaton]

@savvamitrofanov - Does the reasoning for not including git submodule deinit after all, and the addition of the guard file, look correct to you too?