build: make rpath optional

build: make rpath optional

Type: feature

This patch begins enabling support for building VPP as a package for a
Yocto-based embedded Linux distribution. Yocto QA complains about
executables and libraries with an rpath configured, as the paths may not
be valid on the target system. Therefore, this patch adds a new CMake
option named `VPP_SET_RPATH`, which is enabled by default but can be
disabled by a Yocto-based build.

Change-Id: I90e9c4ba07138032b0f76cb72a98e58470885d55
Signed-off-by: Nathan Moos <nmoos@cisco.com>