bug in KBUILD_OUTPUT handling - for relative paths in kselftest

[Tim.Bird]

I found a bug in kselftest KBUILD_OUTPUT handling.

The following works:
$ cd /home/tbird/work/linux
$ export KBUILD_OUTPUT=/home/tbird/work/kbuild
$ yes '' | make localmodconfig
$ make TARGETS=size kselftest

But this doesn't work:
$ cd /home/tbird/work/linux
$ export KBUILD_OUTPUT=../kbuild
$ yes '' | make localmodconfig
$ make TARGETS=size kselftest

I see the following:
make[1]: Entering directory '/home/tbird/work/kbuild'
 make --no-builtin-rules INSTALL_HDR_PATH=$BUILD/usr \
 	ARCH=x86 -C ../../.. headers_install
   INSTALL ../kbuild/kselftest/usr/include
 gcc -static -ffreestanding -nostartfiles -s    get_size.c  -o ../kbuild/kselftest/size/get_size
   /usr/bin/ld: cannot open output file ../kbuild/kselftest/size/get_size: No such file or directory
 collect2: error: ld returned 1 exit status
   ../lib.mk:138: recipe for target '../kbuild/kselftest/size/get_size' failed
 make[3]: *** [../kbuild/kselftest/size/get_size] Error 1
 Makefile:136: recipe for target 'all' failed
 make[2]: *** [all] Error 2
 /home/tbird/work/linux/Makefile:1240: recipe for target 'kselftest' failed
 make[1]: *** [kselftest] Error 2
 make[1]: Leaving directory '/home/tbird/work/kbuild'
 Makefile:179: recipe for target 'sub-make' failed
 make: *** [sub-make] Error 2

This is due to the relative path for KBUILD_OUTPUT being handled incorrectly (I believe)
in tools/testing/selftests/Makefile.

There are these lines in the Makefile, which are responsible for creating the output
directory:
    BUILD_TARGET=$$BUILD/$$TARGET
    mkdir $$BUILD_TARGET -p

But these are executed from working directory tools/testing/selftests,
so the 'size' directory gets created at tools/testing/kbuild/kselftest/size,
instead of /home/tbird/work/kbuild/kselftest/size.

I can add some code to the Makefile to change the assignment of the
variable BUILD, so that if it is a relative path it is relative to $(top_srcdir)
instead of the current directory.   But I wanted to check and make sure that
I'm not  breaking anyone else's workflow.

I'm not sure what the expectation would be for someone who did this:
 $ export KBUILD_OUTPUT=../kbuild ; make  -C tools/testing/selftests run_tests

But I assume if someone is running the kernel's 'make' from the top-level
kernel source directory, and they have a relative KBUILD_OUTPUT directory,
then they want that output directory to be relative to the top-level directory
and not somewhere else.

Should I just code up something for review?

I use relative KBUILD_OUTPUT paths for a number of my kernel build scripts,
and right now these are incompatible with kselftests.

Thanks,
 -- Tim