unix/make: Allow out-of-tree build.

This allows out-of-tree build like:

```sh
$ make -C ports/unix/ BUILD=/tmp/aaa MICROPY_MPYCROSS=~/bin/mpy-cross test
```

Signed-off-by: Yanfeng Liu <[email protected]>

Author: yf13

ports/unix/Makefile

@@ -14,7 +14,7 @@ $(error Invalid VARIANT specified: $(VARIANT_DIR))
 endif
 
 # If the build directory is not given, make it reflect the variant name.
-BUILD ?= build-$(VARIANT)
+BUILD ?= $(abspath ./build-$(VARIANT))
 
 include ../../py/mkenv.mk
 -include mpconfigport.mk
@@ -258,7 +258,7 @@ include $(TOP)/py/mkrules.mk
 
 test: $(BUILD)/$(PROG) $(TOP)/tests/run-tests.py
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))
-	cd $(TOP)/tests && MICROPY_MICROPYTHON=../$(DIRNAME)/$(BUILD)/$(PROG) ./run-tests.py
+	cd $(TOP)/tests && MICROPY_MICROPYTHON=$(BUILD)/$(PROG) ./run-tests.py
 
 test//%: $(BUILD)/$(PROG) $(TOP)/tests/run-tests.py
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))