os: add stubs for Readlink and File.Seek for baremetal targets, with smoke test.

Test currently enabled on pybadge (chosen at random)

TODO:
- enable test on arduino; currently fails with "interp: ptrtoint integer size..." (#2389)
- enable test on nintendoswitch; currently fails with many missing definitions (#2530)

src/os/file_other.go

@@ -62,6 +62,14 @@ func Pipe() (r *File, w *File, err error) {
 	return nil, nil, ErrNotImplemented
 }
 
+func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
+	return 0, ErrNotImplemented
+}
+
+func Readlink(name string) (string, error) {
+	return "", ErrNotImplemented
+}
+
 func tempDir() string {
 	return "/tmp"
 }