fix freertos compile

examples/device/cdc_msc_freertos/Makefile

@@ -1,11 +1,13 @@
 include ../../../tools/top.mk
 include ../../make.mk
 
+FREERTOS_SRC = lib/FreeRTOS/FreeRTOS/Source
+
 INC += \
 	src \
 	$(TOP)/hw \
-	$(TOP)/lib/FreeRTOS/Source/include \
-	$(TOP)/lib/FreeRTOS/Source/portable/GCC/$(FREERTOS_PORT)
+	$(TOP)/$(FREERTOS_SRC)/include \
+	$(TOP)/$(FREERTOS_SRC)/portable/GCC/$(FREERTOS_PORT)
 	
 # Example source
 EXAMPLE_SOURCE += $(wildcard src/*.c)
@@ -13,12 +15,12 @@ SRC_C += $(addprefix $(CURRENT_PATH)/, $(EXAMPLE_SOURCE))
 
 # FreeRTOS source
 SRC_C += \
-	lib/FreeRTOS/Source/list.c \
-	lib/FreeRTOS/Source/queue.c \
-	lib/FreeRTOS/Source/tasks.c \
-	lib/FreeRTOS/Source/timers.c \
-	lib/FreeRTOS/Source/portable/MemMang/heap_4.c \
-	lib/FreeRTOS/Source/portable/GCC/$(FREERTOS_PORT)/port.c \
+	$(FREERTOS_SRC)/list.c \
+	$(FREERTOS_SRC)/queue.c \
+	$(FREERTOS_SRC)/tasks.c \
+	$(FREERTOS_SRC)/timers.c \
+	$(FREERTOS_SRC)/portable/MemMang/heap_4.c \
+	$(FREERTOS_SRC)/portable/GCC/$(FREERTOS_PORT)/port.c \
 
 # FreeRTOS (lto + Os) linker issue
 LDFLAGS += -Wl,--undefined=vTaskSwitchContext