lantiq: sync image filename with boardname

lantiq: sync image filename with boardname

Use <manufacturer>_<modelname> as image name.

Use the BOARD_NAME variable to ensure that the former used boardname is
still used as the subdirectory name for the sysupgrade-tar image, to
not break sysupgrade from earlier versions.

While at it, normalise the image filenames by using only lower case
characters and bin as file extension for sysupgrade images.

Signed-off-by: Mathias Kresin <dev@kresin.me>