kbuild: remove all dummy assignments to obj-

Now kbuild core scripts create empty built-in.o where necessary.
Remove "obj- := dummy.o" tricks.

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

firmware/Makefile

@@ -59,6 +59,3 @@ endif
 
 targets := $(patsubst $(obj)/%,%, \
                                 $(shell find $(obj) -name \*.gen.S 2>/dev/null))
-# Without this, built-in.o won't be created when it's empty, and the
-# final vmlinux link will fail.
-obj- := dummy