mk: install a standard cutomizable tree

mk: install a standard cutomizable tree

The rule "install" follows these conventions:
http://gnu.org/prep/standards/html_node/Directory-Variables.html
http://gnu.org/prep/standards/html_node/DESTDIR.html

The variable sdkdir has been added to the more standards ones,
to configure the directory used with RTE_SDK when using the DPDK makefiles
to build an application.

It is still possible to build DPDK with the "install T=" rule without
specifying any DESTDIR. In such case there is no install, as before.

The old usage of an installed SDK is:
    make -C examples/helloworld RTE_SDK=$(readlink -m $DESTDIR) \
         RTE_TARGET=x86_64-native-linuxapp-gcc
RTE_TARGET can be specified but is useless now with an installed SDK.
The RTE_SDK directory must now point to a different path depending of
the installation.

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