cpack: add deb/rpm packaging to VPP module

cpack: add deb/rpm packaging to VPP module

This allows easy packaging of external projects, including
VOM and sample plugin.

i.e.

$ make -C build-root PLATFORM=vpp TAG=vpp sample-plugin-install

$ ninja -C build-root/build-vpp-native/sample-plugin package

ninja: Entering directory `build-root/build-vpp-native/sample-plugin'
[0/1] Run CPack packaging tool...
CPack: Create package using DEB
CPack: Install projects
CPack: - Install project: sample-plugin
CPack: -   Install component: vpp-plugin-sample
CPack: -   Install component: vpp-plugin-sample-dev
CPack: Create package
CPack: - package: build-root/build-vpp-native/sample-plugin/vpp-plugin-sample_18.10~rc0~413~g1433e9d1-1_amd64.deb generated.
CPack: - package: build-root/build-vpp-native/sample-plugin/vpp-plugin-sample-dev_18.10~rc0~413~g1433e9d1-1_amd64.deb generated.

Change-Id: I6b3ec2433961e53edd976b19659ce214d272c40a
Signed-off-by: Damjan Marion <damarion@cisco.com>