generator: Fix creation of target directory

This attempts to fix a regression introduced in commit
e13aa77 since this commit
the `usr/lib` target directroy is not created if the host system
does not ship any files in `usr/lib` (e.g. Alpine). This causes
initramfs images generated on such systems to not be bootable
as `lib` is a symlink to the non-existend `usr/lib` then.

The problem is the current implementation:

	filepath.Dir(filepath.Join(filepath.Dir(l.src), l.target))

Would return `/usr` for `{"/lib", "usr/lib"}` and hence `/usr/lib`
was never created. I believe this was intended to be:

	filepath.Join(filepath.Dir(l.src), l.target)

This commit changes this accordingly and fixes booting Booster
images on Alpine.

generator/generator.go

@@ -263,7 +263,7 @@ func appendCompatibilitySymlinks(img *Image) error {
 	for _, l := range symlinks {
 		// Ensure that target always exist which may not be the
 		// case if we only install files from /lib or /bin.
-		targetDir := filepath.Dir(filepath.Join(filepath.Dir(l.src), l.target))
+		targetDir := filepath.Join(filepath.Dir(l.src), l.target)
 		if err := img.AppendDirEntry(targetDir); err != nil {
 			return err
 		}