utilize RTE_EAL_PMD_PATH

utilize RTE_EAL_PMD_PATH

This sets RTE_EAL_PMD_PATH to /usr/lib/$(shell dpkg-architecture
-qDEB_HOST_MULTIARCH)/dpdk-pmds.
On creating the pmd library packages those get a symlink from that dir.
This allows DPDK applications to autoload pmd drivers and only rely on
the EAL -d option for those provided in other places (e.g. custom
drivers).
By that one can use testpmd/l2fwd/openvswitch-dpdk and others without
passing -d <full-path-to-pmd.so> to EAL.

Each lib gets an extra relative symlink from the RTE_EAL_PMD_PATH dir to
the lib itself e.g. for librte_vhost:
./usr/lib/x86_64-linux-gnu/dpdk-pmds/
./usr/lib/x86_64-linux-gnu/dpdk-pmds/librte_vhost.so.3 -> ../librte_vhost.so.3

Change-Id: Idf997e58cfa5060961239b3ee0c22de89e0ed1a3
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>