examples: use pkg-config in makefiles

examples: use pkg-config in makefiles

Change the example app Makefiles to query if DPDK is installed and
registered using pkg-config. If so, build directly using pkg-config info,
otherwise fall back to using the original build system with RTE_SDK and
RTE_TARGET

This commit changes the makefiles for the basic examples, i.e. those which
do not have multiple subdirectories underneath the main examples dir.
Examples not covered are:

* ethtool
* multi_process
* performance-thread
* quota_watermark
* netmap_compat
* server_node_efd
* vm_power_manager

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Luca Boccassi <bluca@debian.org>