Bug#844749: live-build: Jessie image with backported kernel (linux-image-4.7.0-0.bpo.1-amd64) doesn't boot

[intrigeri]

Igor Starkov:
>    [    2.974235] FAT-fs (sda1): IO charset ascii not found
>    mount: mounting /dev/sda1 on /live/medium failed: Invalid argument
>       ....  skipped similar lines ....
>    [   63.554664] FAT-fs (sda1): IO charset ascii not found
>    mount: mounting /dev/sda1 on /live/medium failed: Invalid argument

Try adding a "nls_ascii" line (without quotes) in
/etc/initramfs-tools/modules.