kbuild: rpm-pkg: do not hard-code $MAKE in spec file

Currently, $MAKE will expand to the GNU Make program that created the
source RPM. This is problematic if you carry it to a different build
host to run 'rpmbuild' there.

Consider this command:

  $ /path/to/my/custom/make srcrpm-pkg

The spec file in the SRPM will record '/path/to/my/custom/make', which
exists only on that build environment.

To create a portable SRPM, the spec file should avoid hard-coding $MAKE.

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