py/mkrules: Use abspath to find directory for mpy-cross dependency.

Otherwise if the `mpy-cross/build/` directory doesn't exist then
`mpy-cross/build/..` won't work.

Signed-off-by: Damien George <damien@micropython.org>

py/mkrules.mk

@@ -162,7 +162,7 @@ $(HEADER_BUILD):
 ifneq ($(MICROPY_MPYCROSS_DEPENDENCY),)
 # to automatically build mpy-cross, if needed
 $(MICROPY_MPYCROSS_DEPENDENCY):
-	$(MAKE) -C $(dir $@)..
+	$(MAKE) -C $(abspath $(dir $@)..)
 endif
 
 ifneq ($(FROZEN_DIR),)