kbuild: deb-pkg: use dh_listpackages to know enabled packages

Use dh_listpackages to get a list of all binary packages.

With this, debian/control lists which binary packages will be produced.
Previously, ARCH=um listed linux-libc-dev in debian/control, but it
was not generated because each of mkdebian and builddeb independently
maintained the if-conditionals.

Another motivation is to allow scripts/package/builddeb to get the
package name (linux-image-*, etc.) dynamically from debian/control.

This will also allow the BuildProfile to control the generation of
the binary packages.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>