kbuild: rpm-pkg: skip build dependency check on non-rpm systems

Commit 8818039f ("kbuild: add ability to make source rpm buildable
using koji") added the BuildRequires: field.

Checking the build dependency is fine, but one annoyance is that
'make (bin)rpm-pkg' fails on non-rpm systems [1]. For example, Debian
provides rpmbuild via 'apt install rpm', but of course cannot meet the
requirement listed in the BuildRequires: field.

It is possible to pass RPMOPTS=--nodeps to work around it, but it is
reasonable to do it automatically.

If 'rpm -q rpm' fails, it is not an RPM-managed system. (The command
'rpm' is not installed at all, or was installed by other means.)

In that case, pass --nodeps to skip the build dependency check.

[1]: https://lore.kernel.org/linux-kbuild/Y6mkdYQYmjUz7bqV@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com/Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.package

@@ -92,7 +92,8 @@ rpm-pkg srcrpm-pkg binrpm-pkg: kernel.spec
 	+$(strip rpmbuild -b$(build-type) kernel.spec \
 	--define='_topdir $(abspath rpmbuild)' \
 	$(if $(filter a b, $(build-type)), \
-		--target $(UTS_MACHINE)-linux --build-in-place --noprep --define='_smp_mflags %{nil}') \
+		--target $(UTS_MACHINE)-linux --build-in-place --noprep --define='_smp_mflags %{nil}' \
+		$$(rpm -q rpm >/dev/null 2>&1 || echo --nodeps)) \
 	$(if $(filter b, $(build-type)), \
 		--without devel) \
 	$(RPMOPTS))