ath25: fix duplicate LZMA compression

The conversion to the new image building code accidentally caused the kernel
image to get compressed twice, leading to boot failures when kernel and rootfs
are flashed separately.

The sysupgrade images have been unaffected by this. Also restore the elf
kernel build artifact while we're at it.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>