py: Add ability to have frozen persistent bytecode from .mpy files.

The config variable MICROPY_MODULE_FROZEN is now made of two separate
parts: MICROPY_MODULE_FROZEN_STR and MICROPY_MODULE_FROZEN_MPY.  This
allows to have none, either or both of frozen strings and frozen mpy
files (aka frozen bytecode).

py/frozenmod.h

@@ -24,4 +24,10 @@
  * THE SOFTWARE.
  */
 
-mp_lexer_t *mp_find_frozen_module(const char *str, int len);
+enum {
+    MP_FROZEN_NONE,
+    MP_FROZEN_STR,
+    MP_FROZEN_MPY,
+};
+
+int mp_find_frozen_module(const char *str, size_t len, void **data);