composefs: Fix uki boot with grub2 on fedora aarch64

In Fedora 44 (at least), the grub2 EFI binary for aarch64 doesn't
include the chainloader module (but the x84 one does). We insmod this
in our menu files, which means we get this at boot:

```
error: ../../grub-core/fs/fshelp.c:find_file:257:file `/grub2/arm64-efi/chain.mod' not found.
error: ../../grub-core/script/function.c:grub_script_function_find:119:can't find command `chainloader'.
```

In general, we don't know what modules are built in for any particular
bootc image, so to be safe we copy all the modules we're using (that
exist) into /boot/grub2.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

Author: alexlarsson

crates/lib/src/bootc_composefs/boot.rs

@@ -142,6 +142,18 @@ const AUTH_EXT: &str = "auth";
 /// This is relative to the ESP
 pub(crate) const BOOTC_UKI_DIR: &str = "EFI/Linux/bootc";
 
+// These are the modules we insmod in out menu config files
+const GRUB_MODULES_USED: [&str; 2] = ["fat.mod", "chain.mod"];
+
+fn grub_efi_module_dir() -> Option<&'static str> {
+    match std::env::consts::ARCH {
+        "aarch64" => Some("arm64-efi"),
+        "x86_64" => Some("x86_64-efi"),
+        "riscv64" => Some("riscv64-efi"),
+        _ => None,
+    }
+}
+
 /// Directory (relative to the ESP) where systemd-stub looks for UKI addons that apply
 /// to *every* UKI, as opposed to addons scoped to a single UKI (which live alongside
 /// it under [`BOOTC_UKI_DIR`]). Unlike per-UKI addons, these aren't tied to a single
@@ -2158,6 +2170,27 @@ pub(crate) async fn setup_composefs_boot(
             Some(bind_boot_path.as_path()),
         )?;
 
+        if matches!(postfetch.detected_bootloader, Bootloader::Grub) {
+            // The menu config files insmod some modules, these may be either built
+            // into the GRUB EFI binary, or loaded from /boot/grub2.  For example, the
+            // fedora aarch64 grub doesn't include chain.mod. We install all the modules
+            // we can find to avoid potential problems.
+            if let Some(module_dir) = grub_efi_module_dir() {
+                for module in GRUB_MODULES_USED {
+                    let source = format!("usr/lib/grub/{module_dir}/{module}");
+                    if mounted_root.dir().try_exists(&source)? {
+                        let target = format!("boot/grub2/{module_dir}");
+                        root_setup.physical_root.create_dir_all(&target)?;
+                        let target = root_setup.physical_root.open_dir(&target)?;
+                        mounted_root
+                            .dir()
+                            .copy(&source, &target, module)
+                            .with_context(|| format!("Installing GRUB module {module}"))?;
+                    }
+                }
+            }
+        }
+
         // FIXME: Remove this hack once we have support in bootupd
         if matches!(postfetch.detected_bootloader, Bootloader::GrubCC) {
             // bootupctl wrote this under the physical root's real /boot (via