tests/slip: simplified Makefile

tests/slip/Makefile

@@ -10,19 +10,17 @@ USEMODULE += gnrc_slip
 USEMODULE += shell
 USEMODULE += shell_commands
 
-CFLAGS += -I$(CURDIR)
+# set slip parameters to default values if unset
+SLIP_UART     ?= "UART_NUMOF-1"
+SLIP_BAUDRATE ?= 115200
+
+# export slip parameters
+CFLAGS += -DSLIP_UART="UART_DEV($(SLIP_UART))"
+CFLAGS += -DSLIP_BAUDRATE=$(SLIP_BAUDRATE)
 
-ifneq (,$(SLIP_UART))
-  CFLAGS += -DSLIP_UART="UART_DEV($(SLIP_UART))"
-else
-  # set default
-  CFLAGS += -DSLIP_UART="UART_DEV(UART_NUMOF-1)"
-endif
-ifneq (,$(SLIP_BAUDRATE))
-  CFLAGS += -DSLIP_BAUDRATE=$(SLIP_BAUDRATE)
-else
-  # set default
-  CFLAGS += -DSLIP_BAUDRATE=115200
-endif
+# add current directory to the include path. Putting it in CFLAGS will make
+# it go to the beginning, before the standard includes.
+# That way slip_params.h get's included and auto configuration can pick it up.
+CFLAGS += -I$(CURDIR)
 
 include $(RIOTBASE)/Makefile.include