Warn the user about the usage of output/target as the root filesystem

index c526fe1..3e7d520 100644
@@ -269,6 +269,10 @@ LEGAL_LICENSES_TXT=$(LEGAL_INFO_DIR)/licenses.txt
 LEGAL_WARNINGS=$(LEGAL_INFO_DIR)/.warnings
 LEGAL_REPORT=$(LEGAL_INFO_DIR)/README
 
+# Location of a file giving a big fat warning that output/target
+# should not be used as the root filesystem.
+TARGET_DIR_WARNING_FILE=$(TARGET_DIR)/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM
+
 ifeq ($(BR2_CCACHE),y)
 CCACHE:=$(HOST_DIR)/usr/bin/ccache
 BUILDROOT_CACHE_DIR = $(call qstrip,$(BR2_CCACHE_DIR))

@@ -403,6 +407,7 @@ $(BUILD_DIR)/.root:
 			cp -fa $(TARGET_SKELETON)/* $(TARGET_DIR)/; \
 		fi; \
 	fi
+	cp support/misc/target-dir-warning.txt $(TARGET_DIR_WARNING_FILE)
 	-find $(TARGET_DIR) -type d -name CVS -print0 -o -name .svn -print0 | xargs -0 rm -rf
 	-find $(TARGET_DIR) -type f \( -name .empty -o -name '*~' \) -print0 | xargs -0 rm -rf
 	touch $@

index 0f0eae9..b1512dd 100644
@@ -45,6 +45,7 @@ $(BINARIES_DIR)/rootfs.$(1): $(ROOTFS_$(2)_DEPENDENCIES)
 	@$(call MESSAGE,"Generating root filesystem image rootfs.$(1)")
 	$(foreach hook,$(ROOTFS_$(2)_PRE_GEN_HOOKS),$(call $(hook))$(sep))
 	rm -f $(FAKEROOT_SCRIPT)
+	rm -f $(TARGET_DIR_WARNING_FILE)
 	echo "chown -R 0:0 $(TARGET_DIR)" >> $(FAKEROOT_SCRIPT)
 ifneq ($(ROOTFS_DEVICE_TABLES),)
 	cat $(ROOTFS_DEVICE_TABLES) > $(FULL_DEVICE_TABLE)

@@ -57,6 +58,7 @@ endif
 	echo "$(ROOTFS_$(2)_CMD)" >> $(FAKEROOT_SCRIPT)
 	chmod a+x $(FAKEROOT_SCRIPT)
 	$(HOST_DIR)/usr/bin/fakeroot -- $(FAKEROOT_SCRIPT)
+	cp support/misc/target-dir-warning.txt $(TARGET_DIR_WARNING_FILE)
 	- at rm -f $(FAKEROOT_SCRIPT) $(FULL_DEVICE_TABLE)
 	$(foreach hook,$(ROOTFS_$(2)_POST_GEN_HOOKS),$(call $(hook))$(sep))
 ifeq ($$(BR2_TARGET_ROOTFS_$(2)_GZIP),y)

new file mode 100644
index 0000000..b39b759
@@ -0,0 +1,29 @@
+Warning!
+========
+
+This directory does *not* contain the root filesystem that you can use
+on your embedded system. Since Buildroot does not run as root, it
+cannot create device files and set the permissions and ownership of
+files correectly in this directory to make it usable as a root
+filesystem.
+
+For that reason, do *not* use the contents of this directory to mount
+your root filesystem over NFS or copy the contents of this directory
+to a SD card or USB key, thinking it will work as the root filesystem
+for your embedded system. It will simply *not* work.
+
+Instead, if you need a usable root filesystem, please select one of
+the filesystem image formats available in the Buildroot configuration
+interface (make menuconfig or others) in the "Filesystem images"
+sub-menu. If you want to get a filesystem image that you can easily
+extract to your SD card or to some directory exposed through NFS,
+please use the "tar the root filesystem" option. It will generate a
+images/rootfs.tar image in your Buildroot output directory, which you
+can extract as root:
+
+  sudo tar -C /destination/of/extraction -xf images/rootfs.tar
+
+Those image files are created using the contents of the target/
+directory, but there is a post-processing step to create device files
+and set owernship/permissions properly even if Buildroot does not run
+as root.