deb-pkg: Fix cross-building linux-headers package

builddeb generates a control file that says the linux-headers package
can only be built for the build system primary architecture.  This
breaks cross-building configurations.  We should use $debarch for this
instead.

Since $debarch is not yet set when generating the control file, set
Architecture: any and use control file variables to fill in the
description.

Fixes: cd8d60a2 ('kbuild: create linux-headers package in deb-pkg')
Reported-and-tested-by: "Niew, Sh." <shniew@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>