build: add definitions for use as Meson subproject

build: add definitions for use as Meson subproject

To allow other projects to easily use DPDK as a subproject, add in the
necessary dependency definitions. Slightly different definitions are
necessary for static and shared builds, since for shared builds the
drivers should not be linked in, and the internal meson dependency
objects are more complete.

To use DPDK as a subproject fallback i.e. use installed DPDK if present,
otherwise the shipped one, the following meson statement can be used:

libdpdk = dependency('libdpdk', fallback: ['dpdk', 'dpdk_dep'])

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Ben Magistro <koncept1@gmail.com>
Tested-by: Ben Magistro <koncept1@gmail.com>