Revert "brcm2708: fix sdcard image"

This reverts commit 6c2e1ff80f.

GNU gzip does not fail when the image filename already contains the .gz
extension, this is a problem specific to pigz. Revert the commit, as we
now gzip the image twice.

Reported-by: Martin Schleier <drahemmaps@gmx.net>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

target/linux/brcm2708/image/Makefile

@@ -50,8 +50,8 @@ define Device/Default
   FILESYSTEMS := ext4
   KERNEL := kernel-bin | kernel-img
   KERNEL_IMG := kernel.img
-  IMAGES := sdcard.img
-  IMAGE/sdcard.img := boot-img | sdcard-img | gzip | append-metadata
+  IMAGES := sdcard.img.gz
+  IMAGE/sdcard.img.gz := boot-img | sdcard-img | gzip | append-metadata
 endef
 
 define Device/rpi