mpy-cross: Make build independent of extmod directory.

mpy-cross doesn't depend on any code in the extmod directory so completely
exclude it from the build (extmod may still be scanned for qstrs but that
is controlled by py/py.mk).  This speeds up the build a little, and
improves abstraction of this component.

Also, make -I$(BUILD) take precedence over -I$(TOP) in case there are stray
files in the root directory that would be picked up.

mpy-cross/Makefile

@@ -26,8 +26,8 @@ UNAME_S := $(shell uname -s)
 include $(TOP)/py/py.mk
 
 INC += -I.
-INC +=  -I$(TOP)
 INC += -I$(BUILD)
+INC += -I$(TOP)
 
 # compiler settings
 CWARN = -Wall -Werror
@@ -68,7 +68,7 @@ ifneq (,$(findstring mingw,$(COMPILER_TARGET)))
 	SRC_C += ports/windows/fmode.c
 endif
 
-OBJ = $(PY_O)
+OBJ = $(PY_CORE_O)
 OBJ += $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 
 include $(TOP)/py/mkrules.mk

mpy-cross/mphalport.h

-// empty file
+// prevent including extmod/virtpin.h
+#define mp_hal_pin_obj_t