kbuild: rpm-pkg: Restrict manual debug package creation

kbuild: rpm-pkg: Restrict manual debug package creation

Commit 62089b804895 ("kbuild: rpm-pkg: Generate debuginfo package
manually") moved away from the built-in RPM machinery for generating
-debuginfo packages to a more manual way to be compatible with module
signing, as the built-in machinery strips the modules after the
installation process, breaking the signatures.

Unfortunately, prior to rpm 4.20.0, there is a bug where a custom %files
directive is ignored for a -debuginfo subpackage [1], meaning builds
using older versions of RPM (such as on RHEL9 or RHEL10) fail with:

  Checking for unpackaged file(s): /usr/lib/rpm/check-files .../rpmbuild/BUILDROOT/kernel-6.19.0_dirty-1.x86_64
  error: Installed (but unpackaged) file(s) found:
     /debuginfo.list
     /usr/lib/debug/.build-id/09/748c214974bfba1522d434a7e0a02e2fd7f29b.debug
     /usr/lib/debug/.build-id/0b/b96dd9c7d3689d82e56d2e73b46f53103cc6c7.debug
     /usr/lib/debug/.build-id/0e/979a2f34967c7437fd30aabb41de1f0c8b6a66.debug
    ...

To workaround this, restrict the manual debug info package creation
process to when it is necessary (CONFIG_MODULE_SIG=y) and possible (when
using RPM >= 4.20.0). A follow up change will restore the RPM debuginfo
creation process using a separate internal flag to allow the package to
be built in more situations, as RPM 4.20.0 is a fairly recent version
and the built-in -debuginfo generation works fine when module signing is
disabled.

Cc: stable@vger.kernel.org
Fixes: 62089b804895 ("kbuild: rpm-pkg: Generate debuginfo package manually")
Link: https://github.com/rpm-software-management/rpm/commit/49f906998f3cf1f4152162ca61ac0869251c380f
Reported-by: Steve French <smfrench@gmail.com>
Closes: https://lore.kernel.org/CAH2r5mugbrHTwnaQwQiYEUVwbtqmvFYf0WZiLrrJWpgT8iwftw@mail.gmail.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>