kbuild: remove obj-n and lib-n handling

Kconfig never defines CONFIG_* as 'n'.
Now obj-n is only used in firmware/Makefile and it can be
replaced with obj-.  No makefile uses lib-n.

Let's rip off obj-n and lib-n.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/firmware/Makefile b/firmware/Makefile
index 0862d34..e297e1b 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -232,6 +232,6 @@
 
 # Without this, built-in.o won't be created when it's empty, and the
 # final vmlinux link will fail.
-obj-n := dummy
+obj- := dummy
 
 hostprogs-y := ihex2fw