Simplify injecting runtime config

[deitch]

We need a way to simplify changing runtime config when building. Specifically, we should be able to do make installer or make live and have it build with the following alternate runtime parameters:

• Controller URL
• Controller server certificate CA
• Onboard certificate (less important)
• Onboard serial (less important)

The simplest way might be (as discussed):

make live CONTROLLER=https://192.168.7.5:443/ SERVER_CERT=./cert-file.pem

When finished, it should indicate where the onboard cert is, and what its serial will be, if available.

cc @rvs