This patch adds the ability to compile and install the kernel
selftests into the target at /usr/lib/kselftests. The rationale behind
/usr/lib is that the selftests have subdirectories where they are
installed which makes them unsuitable to be placed in /usr/sbin as
this would result in /usr/sbin/kselftests/x/y/z. While the selftests
aren't libraries either, they don't achieve much as a standalone
binary so they can be considered to be a 'library of tests' making
/usr/lib sensible.

The selftests require that the kernel headers be installed into the
kernel build tree as some of the selftests have a hardcoded CFLAGS to
include kernel headers (CFLAGS += -I../../../../usr/include/). This is
most easily achieved by using the make ... headers_install inside the
kernel build dir.

This is likely to be a rarely used debugging/performance feature for
development and unlikely to be used in a production configuration.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas:
 - remove bash as a build dependency, it is only a runtime dependency.
 - fix typo in the Config.in help text, and rewrap
 - add missing 'depends on BR2_USE_MMU' dependency for the comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>