[source] apm821xx: explicitly build the rootfs.img.gz target

[LEDE Commits]

mkresin pushed a commit to source.git, branch master:
https://git.lede-project.org/7c2106696de79d51ce6d06ecd8c43eeade791b10

commit 7c2106696de79d51ce6d06ecd8c43eeade791b10
Author: Christian Lamparter <chunkeey at gmail.com>
AuthorDate: Sun Dec 17 14:59:53 2017 +0100

    apm821xx: explicitly build the rootfs.img.gz target
    
    The commit 87b668765e1
    ("image: when using the new image build code, gzip ext4 images by default")
    
    forced that all targets that select the ext4 as the root filesystem
    to always compress the generated rootfs. This is fine, but this method
    doesn't not allow to append the metadata on a per-target base.
    
    Therefore this patch changes the rootfs image production rule to generate
    the gzip step manually. This way the metadata can be appended at a later
    date.
    
    Signed-off-by: Christian Lamparter <chunkeey at gmail.com>
---
 target/linux/apm821xx/image/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target/linux/apm821xx/image/Makefile b/target/linux/apm821xx/image/Makefile
index ee85292..ebc6a05 100644
--- a/target/linux/apm821xx/image/Makefile
+++ b/target/linux/apm821xx/image/Makefile
@@ -217,11 +217,11 @@ define Device/MyBookLiveDefault
   KERNEL := kernel-bin | dtb | gzip | uImage gzip
   KERNEL_INITRAMFS := kernel-bin | dtb | gzip | uImage gzip
   BOOT_SIZE := 8
-  IMAGES := rootfs.img kernel.dtb
+  IMAGES := rootfs.img.gz kernel.dtb
   DEVICE_DTB := apollo3g.dtb
   FILESYSTEMS := ext4
   IMAGE/kernel.dtb := export-dtb
-  IMAGE/rootfs.img := boot-script | boot-img | hdd-img
+  IMAGE/rootfs.img.gz := boot-script | boot-img | hdd-img | gzip
 endef
 
 define Device/MyBookLiveSingle