makefile: Re-scope VERBOSE=true, update build warnings (#9675)
makefile: Re-scope VERBOSE=true, update build warnings (#9675)

* Set 'only' ALL_TARGETS rather than append

This is functionally no different than before, but it's more correct.

* Re-scope VERBOSE=true

Previously this was only set when the OS was Linux; this was added in
805ade7d3.

* Warn about unsupported OS rather than error

Also:

* Only print the warning when trying to build Nomad
* Print correct list of supported OSes
burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad