Make toolchain build dependent on "flag" file

The toolchain build should be dependent on an actual file to ensure we
don't rebuilt it for every target that goes into the initramfs.

mfs/linux_mfs/Makefile

@@ -39,11 +39,14 @@ clean:
 	rm -rf $(TARGET_BUILD_PATH)
 	rm -rf $(BOOT_PATH)
 
-toolchain:
-	$(MAKE) -C $(SOURCE_PATH)/$@ all
+toolchain: $(BUILDROOT_PATH)/toolchain-built
+
+$(BUILDROOT_PATH)/toolchain-built:
+	$(MAKE) -C $(SOURCE_PATH)/toolchain all
 	$(MAKE) -C $(SOURCE_PATH)/zlib install-sysroot
 	$(MAKE) -C $(SOURCE_PATH)/openssl install
 	$(MAKE) -C $(SOURCE_PATH)/ncurses install
+	touch $(BUILDROOT_PATH)/toolchain-built
 
 $(MODULES): toolchain
 	$(MAKE) -C $(SOURCE_PATH)/$@ all