tests/driver_isl29125: simplified Makefile

tests/driver_isl29125/Makefile

@@ -6,17 +6,12 @@ FEATURES_REQUIRED = periph_i2c
 USEMODULE += isl29125
 USEMODULE += xtimer
 
-ifneq (,$(TEST_ISL29125_I2C))
-  CFLAGS += -DTEST_ISL29125_I2C=$(TEST_ISL29125_I2C)
-else
-  # set random default
-  CFLAGS += -DTEST_ISL29125_I2C=I2C_0
-endif
-ifneq (,$(TEST_ISL29125_IRQ_PIN))
-  CFLAGS += -DTEST_ISL29125_IRQ_PIN=$(TEST_ISL29125_IRQ_PIN)
-else
-  # set random default
-  CFLAGS += -DTEST_ISL29125_IRQ_PIN=GPIO_PIN\(0,0\)
-endif
+# set default device parameters in case they are undefined
+TEST_ISL29125_I2C     ?= I2C_0
+TEST_ISL29125_IRQ_PIN ?= GPIO_PIN\(0,0\)
+
+# export parameters
+CFLAGS += -DTEST_ISL29125_I2C=$(TEST_ISL29125_I2C)
+CFLAGS += -DTEST_ISL29125_IRQ_PIN=$(TEST_ISL29125_IRQ_PIN)
 
 include $(RIOTBASE)/Makefile.include