mk: remove multi-target install

mk: remove multi-target install

The multi-target install create some subdirectories with the target name
which is not standard for a "make install" procedure.

The uninstall procedure cannot be applied properly (without removing
all files in a directory). It would need to pre-compute paths.
As it is a packaging issue, it is removed from the build system capabilities.

The variable BUILD_DIR is also renamed to RTE_OUTPUT used in other files.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Panu Matilainen <pmatilai@redhat.com>