py: Allow to to build MicroPython as a static library.

The whole current port gets slurped into a static lib named
"libmicropython.a". Maybe that's not ideal, but at least something
to start with.

py/mkenv.mk

@@ -51,6 +51,7 @@ LD = $(CROSS_COMPILE)ld
 OBJCOPY = $(CROSS_COMPILE)objcopy
 SIZE = $(CROSS_COMPILE)size
 STRIP = $(CROSS_COMPILE)strip
+AR = $(CROSS_COMPILE)ar
 ifeq ($(MICROPY_FORCE_32BIT),1)
 CC += -m32
 CXX += -m32

py/mkrules.mk

@@ -87,6 +87,9 @@ ifndef DEBUG
 endif
 	$(Q)$(SIZE) $(PROG)
 
+lib: $(OBJ)
+	$(AR) rcs libmicropython.a $(OBJ)
+
 clean: clean-prog
 clean-prog:
 	$(RM) -f $(PROG)