Update Makefile related to virtio-fs

When `make check`, if MOUNT_FOLDER is not exist, then create a new one.

Author: MikazukiHikari

Makefile

@@ -44,9 +44,10 @@ endif
 # virtio-fs
 ENABLE_VIRTIOFS ?= 1
 $(call set-feature, VIRTIOFS)
+MOUNT_FOLDER ?= ./shared
 ifeq ($(call has, VIRTIOFS), 1)
     OBJS_EXTRA += virtio-fs.o
-    OPTS += -s ./shared
+    OPTS += -s $(MOUNT_FOLDER)
 endif
 
 NETDEV ?= tap
@@ -205,7 +206,14 @@ ext4.img:
 	$(Q)dd if=/dev/zero of=$@ bs=4k count=600
 	$(Q)$(MKFS_EXT4) -F $@
 
-check: $(BIN) minimal.dtb $(KERNEL_DATA) $(INITRD_DATA) $(DISKIMG_FILE)
+.PHONY: $(MOUNT_FOLDER)
+$(MOUNT_FOLDER):
+	@if [ ! -d $@ ]; then \
+		echo "Creating mount folder: $@"; \
+		mkdir -p $@; \
+	fi
+
+check: $(BIN) minimal.dtb $(KERNEL_DATA) $(INITRD_DATA) $(DISKIMG_FILE) $(MOUNT_FOLDER)
 	@$(call notice, Ready to launch Linux kernel. Please be patient.)
 	$(Q)./$(BIN) -k $(KERNEL_DATA) -c $(SMP) -b minimal.dtb -i $(INITRD_DATA) -n $(NETDEV) $(OPTS)