ath79: use lzma-loader for ZyXEL NBG6716

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: 5d8ea6d34f9 ("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>