Add Debian packaging
Somewhat based on what the kernel creates with `make deb-pkg` but
modified to make it more like a proper Debian package.
Later on we want to rename things like
linux-image-5.4.0-1-librem5
(that is swap API version and kernel type) but that's not possible since
we have another naming out in the wild for 5.3. That's also the reason
why we have to pick a API version of h1 (to be 'greater' than g<sha1>).
Can be built with:
dpkg-buildpackage -uc -us -B --host-arch=arm64
See also #107
Signed-off-by: Guido Günther <guido.gunther@puri.sm>