armvirt: fix overlay support by updating to f2fs/ext4 based overlays

Current snapshot images doesn't provide images with usable overlay
filesystems:

 mount_root: no usable overlay filesystem found, using tmpfs overlay

So this patch fixes this by updating support to current f2fs/ext4 based
overlays and providing squashfs with proper rootfs
TARGET_ROOTFS_PARTSIZE size.

While at it, I've also added support for optional gzipping of the
images, and renamed the output rootfs image filenames to match the more
common naming pattern $(IMG_PREFIX)-rootfs-$(FS-TYPE).img.

Cc: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/armvirt/Makefile

@@ -17,4 +17,6 @@ KERNEL_PATCHVER:=4.19
 
 include $(INCLUDE_DIR)/target.mk
 
+DEFAULT_PACKAGES += mkf2fs e2fsprogs
+
 $(eval $(call BuildTarget))

target/linux/armvirt/image/Makefile

@@ -19,10 +19,16 @@ define Image/Build/Initramfs
 	)
 endef
 
+define Image/Build/gzip
+	gzip -f9n $(BIN_DIR)/$(IMG_ROOTFS)-$(1).img
+endef
+
+$(eval $(call Image/gzip-ext4-padded-squashfs))
+
 define Image/Build
 	$(call Image/Build/$(1))
-	dd if=$(KDIR)/root.$(1) bs=128k conv=sync | \
-		gzip -9n >$(BIN_DIR)/$(IMG_PREFIX)-root.$(1).gz
+	$(CP) $(KDIR)/root.$(1) $(BIN_DIR)/$(IMG_ROOTFS)-$(1).img
+	$(call Image/Build/gzip/$(1))
 endef
 
 $(eval $(call BuildImage))