Bug#966503: raspi-firmware: /etc/kernel/postinst.d/z50-raspi-firmware should deal with compressed kernels

[Victor Westerhuis]

When using `make bindeb-pkg` just add
`KBUILD_IMAGE=arch/arm64/boot/Image` to your make command line. This
will ensure that the uncompressed kernel image is copied instead of the
gzipped one.