scripts/kconfig: allow building with make 3.80 again

Documentation/Changes still lists this as the minimal required version,
so it ought to remain usable for the time being.

Fixes: d2036f30 ("scripts/kconfig/Makefile: Allow KBUILD_DEFCONFIG to be a target")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michal Marek <mmarek@suse.com>