devtools: add path to additional shared object files

devtools: add path to additional shared object files

Drivers librte_mempool_ring.so and librte_pmd_null.so are loaded by
librte_eal.so when running testpmd.
In Ubuntu Xenial, driver path is installed to RPATH on testpmd. This
allows librte_eal.so to find drivers by using the RPATH.
However, in Ubuntu Bionic, driver path is installed to RUNPATH instead.
The RUNPATH on testpmd is not available by librte_eal.so and therefore
lead to driver load failure:

EAL: Detected 32 lcore(s)
EAL: Detected 1 NUMA nodes
EAL: librte_mempool_ring.so: cannot open shared object file:
					No such file or directory
EAL: FATAL: Cannot init plugins
EAL: Cannot init plugins

Add 'drivers' into LD_LIBRARY_PATH so that testpmd can find and make
use of these shared libraries.

Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Aaron Conole <aconole@redhat.com>