selftests: set the BUILD variable to absolute path

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.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>