kbuild: rpm-pkg: do not force -jN in submake

The spec file always passes %{?_smp_mflags}, but we have two
problems here.

[1] "make -jN rpm-pkg" emits the following warning message:

    make[2]: warning: -jN forced in submake: disabling jobserver mode.

[2] We can not specify the number of jobs that run in parallel.
    Whether we give -jN or not from the top Makefile, the spec file
    always passes ${?_smp_mflags} to the build commands.

${?_smp_mflags} will be useful when we run rpmbuild by hand.  When we
invoke it from Makefile, -jN is propagated down to submake; it should
not be overridden because we want to respect the number of jobs given
by the user.  Set _smp_mflags to empty string in this case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>