Revise HDMI to CSI instructions so they work on top of an existing install

[mtlynch]

We recently were helping a customer in the support forum who had an HDMI to CSI setup, but they had run the installation for HDMI to USB.

We should adjust our HDMI to CSI install instructions so that they work in both scenarios:

• A bare Raspbian system
• A Raspbian system where the user installed TinyPilot Community with the default options