selftests: set the BUILD variable to absolute path

selftests: set the BUILD variable to absolute path

Patch series "selftests: Fix separate output directory builds", v2.

Build of several selftests fail if separate output directory is
specified by the following methods:
1) make -C tools/testing/selftests O=<build_dir>
2) export KBUILD_OUTPUT="build_dir"; make -C tools/testing/selftests

Build fails because of several reasons:
1) The kernel headers aren't found.
2) The path of output objects is wrong and hence unaccessible.

These problems can be solved by:
1) Including the correct path of uapi header files
2) By setting the BUILD variable correctly inside Makefile

Following different build scenarios have been tested after making these
changes to verify that nothing gets broken with these changes:
make -C tools/testing/selftests
make -C tools/testing/selftests/futex
make -C tools/testing/selftests/kvm
make -C tools/testing/selftests/landlock
make -C tools/testing/selftests/net
make -C tools/testing/selftests/net/mptcp
make -C tools/testing/selftests/vm
make -C tools/testing/selftests O=build
make -C tools/testing/selftests o=/opt/build
export KBUILD_OUTPUT="/opt/build"; make -C tools/testing/selftests
export KBUILD_OUTPUT="build"; make -C tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests
cd <any_dir>; make -C <src_path>/tools/testing/selftests O=build

This patch (of 10):

The build of kselftests fails if relative path is specified through
KBUILD_OUTPUT or O=<path> method.  BUILD variable is used to determine the
path of the output objects.  When make is run from other directories with
relative paths, the exact path of the build objects is ambiguous and build
fails.

	make[1]: Entering directory '/home/usama/repos/kernel/linux_mainline2/tools/testing/selftests/alsa'
	gcc     mixer-test.c -L/usr/lib/x86_64-linux-gnu -lasound  -o build/kselftest/alsa/mixer-test
	/usr/bin/ld: cannot open output file build/kselftest/alsa/mixer-test

Set the BUILD variable to the absolute path of the output directory.  Make
the logic readable and easy to follow.  Use spaces instead of tabs for
indentation as if with tab indentation is considered recipe in make.

Link: https://lkml.kernel.org/r/20220119101531.2850400-1-usama.anjum@collabora.com
Link: https://lkml.kernel.org/r/20220119101531.2850400-2-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Andr Almeida <andrealmeid@collabora.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Mickal Salan <mic@digikod.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Cc: Minghao Chi <chi.minghao@zte.com.cn>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>