Enable DPDK shared object build with SPDK shared object build.

When building SPDK with shared objects, we should also build the DPDK
submodule shared libraries. This will make it much easier for peole to
link their applications against a dpdk shared library if the DPDK shared
libraries installed on their system don't have all the symbols that SPDK
needs.

Also, add a linker argument to DPDK to specify that it should look in
the dpdk subdirectory when linking libraries.

Note: this change will not work until the default config options in the
DPDK submodule are updated.

Change-Id: I9546df7b84952eb20fe1ac98e3b3bbd0fcbda3a7
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/463028


Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Jan Kryl <jan.kryl@mayadata.io>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>