commit 
8ce72dc32578 ("selftests: fix headers_install circular dependency")
broke bpf build/test workflow. When KBUILD_OUTPUT is set, bpf objects end
up in KBUILD_OUTPUT build directory instead of in ../selftests/bpf.
The following bpf workflow breaks when it can't find the test_verifier:
cd tools/testing/selftests/bpf; make; ./test_verifier;
Fix it to set OUTPUT only when it is undefined in lib.mk. It didn't need
to be set in the first place.
Fixes: 8ce72dc32578 ("selftests: fix headers_install circular dependency")
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
 
 CC := $(CROSS_COMPILE)gcc
 
 ifeq (0,$(MAKELEVEL))
-    ifneq ($(O),)
-       OUTPUT := $(O)
-    else
-       ifneq ($(KBUILD_OUTPUT),)
-               OUTPUT := $(KBUILD_OUTPUT)
-       else
-               OUTPUT := $(shell pwd)
-               DEFAULT_INSTALL_HDR_PATH := 1
-       endif
+    ifeq ($(OUTPUT),)
+       OUTPUT := $(shell pwd)
+       DEFAULT_INSTALL_HDR_PATH := 1
     endif
 endif
 selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))