[PATCH RFC] Makefile: Export absolute srctree path for out-of-tree builds

[Li Zhijian]

Fixes an issue where out-of-tree kselftest builds fail when building
the BPF and bpftools components. The failure occurs because the top-level
Makefile passes a relative srctree path ('..') to its sub-Makefiles, which
leads to errors in locating necessary files.

For example, the following error is encountered:

```
$ make V=1 O=$build/ TARGETS=hid kselftest-all
...
make -C ../tools/testing/selftests all
make[4]: Entering directory '/path/to/linux/tools/testing/selftests/hid'
make  -C /path/to/linux/tools/testing/selftests/../../../tools/lib/bpf OUTPUT=/path/to/linux/O/kselftest/hid/tools/build/libbpf/ \
            EXTRA_CFLAGS='-g -O0'                                      \
            DESTDIR=/path/to/linux/O/kselftest/hid/tools prefix= all install_headers
make[5]: Entering directory '/path/to/linux/tools/lib/bpf'
...
make[5]: Entering directory '/path/to/linux/tools/bpf/bpftool'
Makefile:127: ../tools/build/Makefile.feature: No such file or directory
make[5]: *** No rule to make target '../tools/build/Makefile.feature'.  Stop.
```

To resolve this, the srctree is exported as an absolute path (abs_srctree)
when performing an out-of-tree build. This ensures that all sub-Makefiles
have the correct path to the source tree, preventing directory resolution
errors.

Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
---
Request for Additional Testing

We welcome all contributors and CI systems to test this change thoroughly.
In theory, this change should not affect in-tree builds. However, to ensure
stability and compatibility, we encourage testing across different
configurations.

What has been tested?
- out-of-tree kernel build
- out-of-tree kselftest-all
---
 Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index e5b8a8832c0c..36e65806bb5e 100644
--- a/Makefile
+++ b/Makefile
@@ -275,7 +275,8 @@ else ifeq ($(srcroot)/,$(dir $(CURDIR)))
     srcroot := ..
 endif
 
-export srctree := $(if $(KBUILD_EXTMOD),$(abs_srctree),$(srcroot))
+srctree := $(if $(KBUILD_EXTMOD),$(abs_srctree),$(srcroot))
+export srctree := $(if $(building_out_of_srctree),$(abs_srctree),$(srctree))
 
 ifdef building_out_of_srctree
 export VPATH := $(srcroot)
-- 
2.44.0