py: Make print() accept "file" argument, and actually print to stream.
And not system printf(), like it was before. For this, move pfenv_printf()
from stmhal port to py/.
diff --git a/py/builtin.h b/py/builtin.h
index 361cef6..425cfec 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -81,5 +81,10 @@
extern const mp_obj_module_t mp_module_sys;
extern const mp_obj_module_t mp_module_gc;
+struct _dummy_t;
+extern struct _dummy_t mp_sys_stdin_obj;
+extern struct _dummy_t mp_sys_stdout_obj;
+extern struct _dummy_t mp_sys_stderr_obj;
+
// extmod modules
extern const mp_obj_module_t mp_module_uctypes;