kbuild: simplify command line creation in scripts/mkmakefile

Assuming we never invoke the generated Makefile from outside of
the $(objtree) directory, $(CURDIR) points to the absolute path
of $(objtree).

BTW, 'lastword' is natively supported by GNU Make 3.81+, which
is the current requirement for building the kernel.

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