ath79: add empty squashfs-lzma filesystem

ath79: add empty squashfs-lzma filesystem

The filesystem is currently created on every build to trick the boot
loader of some FRITZ! devices into accepting the image. Sadly the
resulting squashfs-lzma filesystem is not reproducible. To fix this,
create a squashfs filesystem once and include it into the repository.

Creation happend as shown below

    rm -rf empty_dir
    mkdir empty_dir
    ./staging_dir/host/bin/mksquashfs-lzma \
    	empty_dir/ empty-squashfs-lzma \
    	-noappend -root-owned -be -nopad -b 65536 -fixed-time 0

Signed-off-by: Paul Spooren <mail@aparcar.org>