buildroot: dpkg-deb: don't hardcode linux deb package name

Linux debian package name is dynamically generated, and it is available
in debian/image.files file in linux kernel after the build, so don't
hardcode it and read it from debian/image.files.

This commit fixes the below build issue during buildroot build,

dpkg-deb: error: failed to read archive '<path>/linux/out/aemfvp-a-rme/
debs/linux-image-6.15.0-rc1-g78b23c56de79-dirty_6.15.0-rc1-g78b23c56de79
-2_arm64.deb': No such file or directory

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>