mk: add "make examples" target in root makefile

mk: add "make examples" target in root makefile

It is now possible to build all projects from the examples/ directory
using one command from root directory.

Some illustration of what is possible:

- build examples in the DPDK tree for one target

  # install the x86_64-default-linuxapp-gcc in
  # ${RTE_SDK}/x86_64-default-linuxapp-gcc directory
  user@droids:~/dpdk.org$ make install T=x86_64-default-linuxapp-gcc
  # build examples for this new installation in
  # ${RTE_SDK}/examples directory
  user@droids:~/dpdk.org$ make examples T=x86_64-default-linuxapp-gcc

- build examples outside DPDK tree for several targets

  # install all targets matching x86_64-*-linuxapp-gcc in
  # ${RTE_SDK}/x86_64-*-linuxapp-gcc directories
  user@droids:~/dpdk.org$ make install T=x86_64-*-linuxapp-gcc
  # build examples for these installations in /tmp/foobar
  user@droids:~/dpdk.org$ make examples T=x86_64-*-linuxapp-gcc O=/tmp/foobar

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>