ltp-testsuite: change install prefix

ltp-testsuite: change install prefix

ltp-testsuite installs some of its files directly at its specified
prefix, which by default in Buildroot is /usr for autotools packages.
This is currently leaving scripts and its internal directories directly
installed in /usr, such as in:

  # ls /usr
  IDcheck.sh       lib64            runltplite.sh    share
  Version          libexec          runtest          testcases
  bin              runalltests.sh   sbin             testscripts
  lib              runltp           scenario_groups  ver_linux

It also seems to be not feasible to try to install all of its testcases
to standard locations such as /usr/bin as may contain hundreds of
binaries and may not be able to find them all if we change their install
locations. Therefore, it is better if ltp-testsuite installs its tree to
its own self-contained subdirectory.
Upstream instructions recommend that the install path be /opt/ltp,
however it seems that installing things to /opt is not a very common
Buildroot practice.
The proposal then is to install it to /usr/lib/ltp-testsuite and so it
can be run directly from there.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>