initramfs: follow symlinks on host file system.

Should also fix u-root#842.

pkg/cpio/fs_unix.go

@@ -210,6 +210,27 @@ func inode(i Info) (Info, bool) {
 	return i, false
 }
 
+func GetFollowedRecord(path string) (Record, error) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return Record{}, err
+	}
+
+	sys := fi.Sys().(*syscall.Stat_t)
+	info, done := inode(sysInfo(path, sys))
+
+	switch fi.Mode() & os.ModeType {
+	case 0: // Regular file.
+		if done {
+			return Record{Info: info}, nil
+		}
+		return Record{Info: info, ReaderAt: NewLazyFile(path)}, nil
+
+	default:
+		return StaticRecord(nil, info), nil
+	}
+}
+
 func GetRecord(path string) (Record, error) {
 	fi, err := os.Lstat(path)
 	if err != nil {

pkg/uroot/initramfs/files.go

@@ -189,7 +189,7 @@ func (af *Files) WriteTo(w Writer) error {
 //
 // If `src` is a directory, its children will be added to the archive as well.
 func writeFile(w Writer, src, dest string) error {
-	record, err := cpio.GetRecord(src)
+	record, err := cpio.GetFollowedRecord(src)
 	if err != nil {
 		return err
 	}