all: Add py/mphal.h and use it in all ports.

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

esp8266/main.c

@@ -33,10 +33,10 @@
 #include "py/runtime.h"
 #include "py/stackctrl.h"
 #include "py/frozenmod.h"
+#include "py/mphal.h"
 #include "py/gc.h"
 #include "pyexec.h"
 #include "gccollect.h"
-#include MICROPY_HAL_H
 #include "user_interface.h"
 
 STATIC char heap[16384];