[PATCH bpf-next v2] selftests/bpf: avoid generating untracked files when running bpf selftests

[Jiayuan Chen <mrpre@163.com>]

Currently, when we run the BPF selftests with the following command:
'make -C tools/testing/selftests TARGETS=bpf SKIP_TARGETS=""'

The command generates untracked files and directories with make version
less than 4.4:
'''
Untracked files:
  (use "git add <file>..." to include in what will be committed)
	tools/testing/selftests/bpfFEATURE-DUMP.selftests
	tools/testing/selftests/bpffeature/
'''
We lost slash after word "bpf".

The reason is slash appending code is as follow:
'''
OUTPUT := $(OUTPUT)/
$(eval include ../../../build/Makefile.feature)
OUTPUT := $(patsubst %/,%,$(OUTPUT))
'''

This way of assigning values to OUTPUT will never be effective for the
variable OUTPUT provided via the command argument [1] and bpf makefile
is called from parent Makfile(tools/testing/selftests/Makefile) like:
'''
all:
  ...
	$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET
'''

According to GNU make, we can use override Directive to fix this issue [2].

[1]: https://www.gnu.org/software/make/manual/make.html#Overriding
[2]: https://www.gnu.org/software/make/manual/make.html#Override-Directive
Fixes: dc3a8804d790 ("selftests/bpf: Adapt OUTPUT appending logic to lower versions of Make")

Signed-off-by: Jiayuan Chen <mrpre@163.com>

---
v1->v2: fix patchwork check fail.
---
---
 tools/testing/selftests/bpf/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 9e870e519c30..eb4d21651aa7 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -202,9 +202,9 @@ ifeq ($(shell expr $(MAKE_VERSION) \>= 4.4), 1)
 $(let OUTPUT,$(OUTPUT)/,\
 	$(eval include ../../../build/Makefile.feature))
 else
-OUTPUT := $(OUTPUT)/
+override OUTPUT := $(OUTPUT)/
 $(eval include ../../../build/Makefile.feature)
-OUTPUT := $(patsubst %/,%,$(OUTPUT))
+override OUTPUT := $(patsubst %/,%,$(OUTPUT))
 endif
 endif
 
-- 
2.43.5