rework make freertos port handling

this allows ports to specify a freertos port outside the FreeRTOS-Kernel lib directory, which would otherwise not be possible

Signed-off-by: Rafael Silva <rafaelsilva@ajtec.pt>

hw/bsp/lpcxpresso51u68/board.mk

@@ -43,7 +43,7 @@ SRC_S += $(MCU_DIR)/gcc/startup_LPC51U68.S
 LIBS += $(TOP)/$(MCU_DIR)/gcc/libpower.a
 
 # For freeRTOS port source
-FREERTOS_PORT = ARM_CM0
+FREERTOS_PORT = $(FREERTOS_PORT_PATH)/ARM_CM0
 
 JLINK_DEVICE = LPC51U68
 PYOCD_TARGET = LPC51U68