mkfs-util: Add support to populate vfat without mounting using mcopy

mkfs.vfat doesn't support specifying a root directory to bootstrap
the filesystem from (see https://github.com/dosfstools/dosfstools/issues/183).
Instead, we can use the mcopy tool from the mtools package to copy
files into the vfat filesystem after creating it without needing to
mount the vfat filesystem.

test/TEST-58-REPART/test.sh

@@ -15,6 +15,7 @@ test_append_files() {
         if command -v openssl >/dev/null 2>&1; then
             inst_binary openssl
         fi
+        inst_binary mcopy
         instmods dm_verity =md
         generate_module_dependencies
         image_install -o /sbin/mksquashfs

test/test-functions

@@ -1310,6 +1310,11 @@ install_missing_libraries() {
     inst_simple "${path}/engines-3/capi.so" || true
     inst_simple "${path}/engines-3/loader_attic.so" || true
     inst_simple "${path}/engines-3/padlock.so" || true
+
+    # Binaries from mtools depend on the gconv modules to translate between codepages. Because there's no
+    # pkg-config file for these, we copy every gconv/ directory we can find in /usr/lib and /usr/lib64.
+    # shellcheck disable=SC2046
+    inst_recursive $(find /usr/lib* -name gconv 2>/dev/null)
 }
 
 cleanup_loopdev() {