main: detect specific serial port IDs based on USB vid/pid

This makes it possible to flash a board even when there are multiple
different kinds of boards attached, e.g. an Arduino Uno and a Circuit
Playground Express. You can find the VID/PID pair in several ways:

 1. By running `lsusb` before and after attaching the board and looking
    at the new USB device.
 2. By grepping for `usb_PID` and `usb_VID` in the TinyGo source code.
 3. By checking the Arduino IDE boards.txt from the vendor.

Note that one board may have multiple VID/PID pairs:

  * The bootloader and main program may have a different PID, so far
    I've seen that the main program generally has the bootloader PID
    with 0x8000 added.
  * The software running on the board may have an erroneous PID, for
    example from a different board. I've seen this happen a few times.
  * A single board may have had some revisions which changed the PID.
    This is particularly true for the Arduino Uno.

As a fallback, if the given VID/PID pair isn't found, the whole set of
serial ports will be used.

There are many boards which I haven't included yet simply because I
couldn't test them.

compileopts/target.go

@@ -45,6 +45,7 @@ type TargetSpec struct {
 	FlashCommand     string   `json:"flash-command"`
 	GDB              []string `json:"gdb"`
 	PortReset        string   `json:"flash-1200-bps-reset"`
+	SerialPort       []string `json:"serial-port"` // serial port IDs in the form "acm:vid:pid" or "usb:vid:pid"
 	FlashMethod      string   `json:"flash-method"`
 	FlashVolume      string   `json:"msd-volume-name"`
 	FlashFilename    string   `json:"msd-firmware-name"`