stages(grub2): allow pulling efi binaries from alternative efi roots

The EFI binaries are currently pulled from a hardcoded path in the
buildroot. When moving to containers as buildroots this will no
longer work as they have an alternative layout. This is an easy
"fix" - make the location of the `EFI/` directory configurable.

This allows us set `efi_src_dir` to `/usr/lib/bootupd/updates/EFI/`
and keep our existing `bootc-image-builder` workflow.

Note that this may actually not be the desired solution and instead
we want the new `bootupd`: https://github.com/osbuild/osbuild/pull/1519

stages/test/test_grub2.py

@@ -0,0 +1,32 @@
+#!/usr/bin/python3
+
+import os.path
+
+from osbuild.testutil import make_fake_tree
+from osbuild.testutil.imports import import_module_from_path
+
+
+def test_grub2_copy_efi_data(tmp_path):
+    stage_path = os.path.join(os.path.dirname(__file__), "../org.osbuild.grub2")
+    stage = import_module_from_path("stage", stage_path)
+
+    fake_efi_src_dir = tmp_path / "fake-efi/EFI"
+    make_fake_tree(fake_efi_src_dir, {
+        "fedora/a.shim": "fake shim",
+        "BOOT/BOOTX64.EFI": "I'm not real",
+    })
+
+    test_options = {
+        "rootfs": {
+            "label": "my-rootfs",
+        },
+        "uefi": {
+            "install": True,
+            "efi_src_dir": fake_efi_src_dir,
+            "vendor": "fedora",
+        },
+    }
+    fake_tree = tmp_path / "tree"
+    stage.main(fake_tree, test_options)
+    assert (fake_tree / "boot/efi/EFI/fedora/a.shim").exists()
+    assert (fake_tree / "boot/efi/EFI/BOOT/BOOTX64.EFI").exists()