We don't really have a nice way to say "compile this when CONFIG_FOO
is y, don't otherwise".

Alternatives are:

obj-$(subst m,,$(CONFIG_FOO)) := foo.o

or

obj-$(CONFIG_FOO)	:= foo.o
obj-m :=

or

obj-y			:= do_foo.o
do_foo-$(CONFIG_FOO)	:= foo.o

I chose the last one, though I'm not particularly happy with either.