While the new build scripts first introduced when we migrated the development branch to jessie made things much simpler for developers and for people who just want to build the latest VyOS image from source, building an image even simply with a package available from Debian Jessie repos but not present in the VyOS package set by default was still quite an ordeal for a person not familiar with live-build and the structure of our build scripts.
Well, until now. Yesterday I've added ./configure script options that should allow everyone to build a custom image without ever touching the plumbing of the build scripts.
The simplest example, building an image with packages available in Debian Jessie:
./configure --custom-packages "bsdgames robotfindskitten" sudo make iso
A more interesting example, adding a package from a third party repo signed with its own key. In this case, salt-minion:
wget https://repo.saltstack.com/apt/debian/8/amd64/2017.7/SALTSTACK-GPG-KEY.pub ./configure --custom-apt-entry "deb http://repo.saltstack.com/apt/debian/8/amd64/2017.7 jessie main" --custom-packages "salt-minion" --custom-apt-key ./SALTSTACK-GPG-KEY.pub sudo make iso
Of course it doesn't guarantee that your image will build or work, but at least it will get you to the debugging phase faster,