travis: Fix build with --enable-shared and DPDK 2.1.

travis: Fix build with --enable-shared and DPDK 2.1.

When building OVS with --enable-shared, -fPIC should be used in DPDK
CFLAGS.  We used to add a custom option for this (CONFIG_RTE_BUILD_FPIC)
to the DPDK configuration, right after CONFIG_RTE_LIBNAME.

Since CONFIG_RTE_LIBNAME has been removed, it seems simpler to add our
custom option at the end of the file.

Furthermore, since vhost support is enabled by default in DPDK 2.1 and
vhost-user is OVS primary target, there's no need to customize the vhost
related option anymore.

Tested-at: https://travis-ci.org/ddiproietto/ovs/builds/79451461

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@nicira.com>