[PATCH] tools: gpio: fix building from the top level with O=

[Sasha Levin]

"make O=<dir> tools/gpio" fails:

  Makefile:27: /tools/build/Makefile.include: No such file or directory

tools/Makefile clears srctree, and the gpio Makefile only works it out
again when building_out_of_srctree is not set, which the top-level
Makefile sets for O= builds.

Also work out srctree when it is empty, as tools/bpf/Makefile does
since commit a89b2cbf71d64 ("tools, bpf: Fix build for 'make -s
tools/bpf O=<dir>'").

Fixes: 4a6a6f5c4aeed ("tools: gpio: Use !building_out_of_srctree to determine srctree")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 tools/gpio/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/gpio/Makefile b/tools/gpio/Makefile
index 342e056c8c665..1cd7636b9cac5 100644
--- a/tools/gpio/Makefile
+++ b/tools/gpio/Makefile
@@ -7,7 +7,13 @@ bindir ?= /usr/bin
 # isn't set and when invoked from selftests build, where srctree
 # is set to ".". building_out_of_srctree is undefined for in srctree
 # builds
+ifeq ($(srctree),)
+update_srctree := 1
+endif
 ifndef building_out_of_srctree
+update_srctree := 1
+endif
+ifeq ($(update_srctree),1)
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))
 srctree := $(patsubst %/,%,$(dir $(srctree)))
 endif
-- 
2.53.0