ath79: use lzma-loader for ZyXEL NBG6716

Since gzip-compressed kernel image stopped fitting on 4MB kernel
partition on the device, use lzma-loader wrapping LZMA-compressed
kernel. This yields bootable device once again, and saves a very
substantial amount of space, the kernel size decreasing from about 4.4MB
to about 2.5MB for 5.10 kernel. This avoids changing of the flash layout
for the device.

While at that, reactivate the build for the device.

Fixes: 5d8ea6d34f ("ath79: Deactivate ZyXEL NBG6716 by default")
Cc: André Valentin <avalentin@marcant.net>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>

target/linux/ath79/image/nand.mk

@@ -350,8 +350,9 @@ define Device/zyxel_nbg6716
   KERNEL_SIZE := 4096k
   BLOCKSIZE := 128k
   PAGESIZE := 2048
-  KERNEL := kernel-bin | append-dtb | uImage none | zyxel-buildkerneljffs | \
-	check-size 4096k
+  LOADER_TYPE := bin
+  KERNEL := kernel-bin | append-dtb | lzma | loader-kernel | uImage none | \
+	zyxel-buildkerneljffs | check-size 4096k
   IMAGES := sysupgrade.tar sysupgrade-4M-Kernel.bin factory.bin
   IMAGE/sysupgrade.tar/squashfs := append-rootfs | pad-to $$$$(BLOCKSIZE) | \
 	sysupgrade-tar rootfs=$$$$@ | append-metadata
@@ -360,6 +361,5 @@ define Device/zyxel_nbg6716
   IMAGE/factory.bin := append-kernel | pad-to $$$$(KERNEL_SIZE) | append-ubi | \
 	zyxel-factory
   UBINIZE_OPTS := -E 5
-  DEFAULT := n
 endef
 TARGET_DEVICES += zyxel_nbg6716