Earlier attempts to get "make O=build kselftest-all" to work were
not successful as they made undesirable changes to some functions
in the top-level Makefile. This series takes a different
approach by removing the root cause of the problem within
kselftest, which is when the sub-Makefile tries to install kernel
headers "backwards" by calling make with the top-level Makefile.
The actual issue comes from the fact that $(srctree) is ".." when
building in a sub-directory with "O=build" which then obviously
makes "-C $(top_srcdir)" point outside of the real source tree.
With this series, the generic kselftest targets work as expected
from the top level with or without a build directory e.g.:
$ make kselftest-all
$ make O=build kselftest-all
Then in order to build using the sub-Makefile explicitly, the
headers have to be installed first. This is arguably a valid
requirement to have when building a tool from a sub-Makefile.
For example, "make -C tools/testing/nvdimm/" fails in a similar
way until <asm/rwonce.h> has been generated by a kernel build.
v2: replace headers_install with headers