[target][dartuino] properly mark files as needing float

Some routines inside the sensor bus code in dartuino need floating
point, so put those in MODULE_FLOAT_SRCS.

Also mark app/accellerometer as needing float.

target/dartuinoP0/rules.mk

@@ -28,12 +28,18 @@ GLOBAL_INCLUDES += $(LOCAL_DIR)/include
 
 MODULE_SRCS += \
     $(LOCAL_DIR)/init.c \
-    $(LOCAL_DIR)/sensor_bus.c \
-    $(LOCAL_DIR)/usb.c \
+    $(LOCAL_DIR)/usb.c
 
 MODULE_DEPS += \
     dev/usb
 
+ifneq ($(ENABLE_DARTUINO_SENSOR_BUS),)
+MODULE_FLOAT_SRCS += \
+    $(LOCAL_DIR)/sensor_bus.c
+GLOBAL_DEFINES += \
+    ENABLE_SENSORBUS=1
+endif
+
 ifneq ($(DISPLAY_PANEL_TYPE),)
 
 MODULE_SRCS += \