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>
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