OS: R72/R73

BT test setup today that includes chameleond running on a Fizz host is a bit ad-hoc right now. chameleon is manually built and deployed via make, make install and scp-ing the files, as opposed to emerge/ebuild-based process that all other Chrome OS test packages use. 
It appears that the emerge-based packaging was chucked out because of cross-compile issues. 

I started looking at this with two CLs here: 
1. https://chromium-review.googlesource.com/c/chromiumos/overlays/board-overlays/+/1373275
2. https://chrome-internal-review.googlesource.com/c/chromeos/overlays/chromeos-overlay/+/728870

Each has problems - for (2) we want a public overlay. For (1), we want to define what builds will contain chameleon. Apparently having this on all Fizz boards isnt a good idea. I am not sure if that means we need a "bt_testbed" profile similar to what wifi_testbed_ap does, or if there's a better way to add a USE flag and very restrictively use it (i.e only on special test images). 

But we do need to sort this out if we want wifi/bt testing to happen together.