kbuild: get rid of duplication in *.mod files

It is allowed to add the same objects multiple times to obj-y / obj-m:

obj-y += foo.o foo.o foo.o
obj-m += bar.o bar.o bar.o

It is also allowed to add the same objects multiple times to a composite
module:

obj-m += foo.o
foo-y := foo1.o foo2.o foo2.o foo1.o

This flexibility is useful because the same object might be selected by
different CONFIG options, like this:

obj-m += foo.o
foo-y := foo1.o
foo-$(CONFIG_FOO_X) += foo2.o
foo-$(CONFIG_FOO_Y) += foo2.o

The duplicated objects are omitted at link time. It works naturally in
Makefiles because GNU Make removes duplication in $^ without changing
the order.

It is working well, almost...

A small flaw I notice is, *.mod contains duplication in such a case.

This is probably not a big deal. As far as I know, the only small
problem is scripts/mod/sumversion.c parses the same file multiple
times.

I am fixing this because I plan to reuse *.mod for other purposes,
where the duplication can be problematic.

The code change is quite simple. We already use awk to drop duplicated
lines in modules.order (see cmd_modules_order in the same file).
I copied the code, but changed RS to use spaces as record separators.

I also changed the file format to list one object per line.

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