ath79: image: pad kernel for Adtran/Bluesocket devices

ath79: image: pad kernel for Adtran/Bluesocket devices

It has been reported that using the sysupgrade-tar image will trigger
"lzma_decode failed error". The RedBoot bootloader always loads data
from flash till block size boundary, so if there's no padding it'll also
load the beginning of rootfs, and it seems that lzma_decoder can't handle
that garbage data. Previously the script creating combined-image
silently padded the kernel and rootfs, but since sysupgrade-tar doesn't,
pad the default kernel image.

Fixes: 900330f ("ath79: image: retire combined-image for Adtran/Bluesocket
devices")
Cc: Brian Gonyer <bgonyer@gmail.com>
Cc: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>