Console logging output contains garbage when device link is enabled

When the platform is built with device link enabled, the console logging output contains garbage when viewing it through minicom / Putty / Tera Term and pressing any button.

With Putty it happens even without pressing any button, unless Putty is explicitly configured to disable flow control, and to turn off "keepalives". Otherwise Putty will also send null packets over the serial console.

Any input received will cause the ./platform/common/devicelink.cpp to enable the device link protocol, wrapping all console logging output in minihdlc frames and protocol buffers.

It would be better if the device link protocol was only activated once a valid device link message was received on the device, instead of simply by receiving any arbitrary data.