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/fomu/family.mk

@@ -19,7 +19,7 @@ INC += \
 	$(TOP)/$(FAMILY_PATH)/include
 
 # For freeRTOS port source
-FREERTOS_PORT = RISC-V
+FREERTOS_PORT = $(FREERTOS_PORT_PATH)/RISC-V
 
 # flash using dfu-util
 $(BUILD)/$(PROJECT).dfu: $(BUILD)/$(PROJECT).bin