UEFI boot: make sure efivarfs loaded in initrd

The efivarfs is needed in order to GPT root partition discovery work.
Without efivarfs initrd won't be able to switch to the real root.

Add the module regardless of hostonly or distro configuration because
dracut takes into account info from the build host and not the target
distro. Adding a module that's already included in dracut's list makes
no harm.

Fixes #562

mkosi/__init__.py

@@ -1419,6 +1419,10 @@ def configure_dracut(args: CommandLineArguments, root: str) -> None:
     ):
         with open(os.path.join(dracut_dir, "30-mkosi-uefi-stub.conf"), "w") as f:
             f.write("uefi_stub=/usr/lib/systemd/boot/efi/linuxx64.efi.stub\n")
+
+    # efivarfs must be present in order to GPT root discovery work
+    if args.esp_partno is not None:
+        with open(os.path.join(dracut_dir, "30-mkosi-efivarfs.conf"), "w") as f:
             f.write('add_drivers+=" efivarfs "\n')