tests/driver_hdc1000: simplified Makefile

tests/driver_hdc1000/Makefile

@@ -6,17 +6,12 @@ FEATURES_REQUIRED = periph_i2c
 USEMODULE += hdc1000
 USEMODULE += xtimer
 
-ifneq (,$(TEST_HDC1000_I2C))
-  CFLAGS += -DTEST_HDC1000_I2C=$(TEST_HDC1000_I2C)
-else
-  # set random default
-  CFLAGS += -DTEST_HDC1000_I2C=I2C_0
-endif
-ifneq (,$(TEST_HDC1000_ADDR))
-  CFLAGS += -DTEST_HDC1000_ADDR=$(TEST_HDC1000_ADDR)
-else
-  # set random default
-  CFLAGS += -DTEST_HDC1000_ADDR=0x43
-endif
+# set default device parameters in case they are undefined
+TEST_HDC1000_I2C  ?= I2C_0
+TEST_HDC1000_ADDR ?= 0x43
+
+# export parameters
+CFLAGS += -DTEST_HDC1000_I2C=$(TEST_HDC1000_I2C)
+CFLAGS += -DTEST_HDC1000_ADDR=$(TEST_HDC1000_ADDR)
 
 include $(RIOTBASE)/Makefile.include