tools/bpf: permit selftests/bpf to be built in a different directory

Fix a couple of issues at tools/testing/selftests/bpf/Makefile so
the following command
   make -C tools/testing/selftests/bpf OUTPUT=/home/yhs/tmp
can put the built results into a different directory.

Also add the built binary test_tcpbpf_user in the .gitignore file.

Fixes: 6882804c ("selftests/bpf: add a test for overlapping packet range checks")
Fixes: 9d1f1594 ("bpf: move cgroup_helpers from samples/bpf/ to tools/testing/selftesting/bpf/")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/.gitignore

@@ -8,5 +8,6 @@ fixdep
 test_align
 test_dev_cgroup
 test_progs
+test_tcpbpf_user
 test_verifier_log
 feature

tools/testing/selftests/bpf/Makefile

@@ -27,7 +27,7 @@ TEST_PROGS := test_kmod.sh test_xdp_redirect.sh test_xdp_meta.sh \
 
 include ../lib.mk
 
-BPFOBJ := $(OUTPUT)/libbpf.a $(OUTPUT)/cgroup_helpers.c
+BPFOBJ := $(OUTPUT)/libbpf.a cgroup_helpers.c
 
 $(TEST_GEN_PROGS): $(BPFOBJ)
 
@@ -58,7 +58,7 @@ CLANG_FLAGS = -I. -I./include/uapi -I../../../include/uapi \
 $(OUTPUT)/test_l4lb_noinline.o: CLANG_FLAGS += -fno-inline
 $(OUTPUT)/test_xdp_noinline.o: CLANG_FLAGS += -fno-inline
 
-%.o: %.c
+$(OUTPUT)/%.o: %.c
 	$(CLANG) $(CLANG_FLAGS) \
 		 -O2 -target bpf -emit-llvm -c $< -o - |      \
 	$(LLC) -march=bpf -mcpu=$(CPU) -filetype=obj -o $@