scripts/package/Makefile: rpmbuild is needed for rpm targets

Before rpm release 4.1, in 2002, either the rpm command or the
rpmbuild command could be used in the rpm-pkg or binrpm-pkg targets,
and the Makefile chose the rpm command if the rpmbuild command wasn't
found.

After release 4.1, however, the rpm command could no longer be used in
place of the rpmbuild command.  As the rpmbuild command is not
installed by default, this can lead to failures with the rpm-pkg and
binrpm-pkg targets:

rpm --define "_builddir ." --target \
    x86_64 -bb ./binkernel.spec
    rpm --target: unknown option
    scripts/package/Makefile:60: recipe for target 'binrpm-pkg' failed

Change the Makefile to use rpmbuild unconditionally to avoid this.

Signed-off-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>