samples: usb: Make each sample have its own USB product string

Each USB sample, when compiled, will provide a different
USB product string. A "nice to have" feature when trying
out the USB samples (they will show up differently in
`lsusb` or `dmesg`), and also illustrates how to set up
custom USB constants in the config files.

Requires #6589.

Signed-off-by: Iván Sánchez Ortega <ivan@sanchezortega.es>