Add whitelist of serial USB devices.

If you connect a USB serial adapter to the system, it
will automatically be checked if it is a modem.
We do that by sending AT commands to them and wait for a
response. Some devices that are not a modem may get confused
by that, so we whitelist them here.
modem: Wait a moment after initializing.

Some UMTS modems need some extra seconds to initialize
themselves after reset, so we wait a short moment...