This prefix isn't actually used and only adds noise, so remove it.
It may have been useful on Linux that makes a distinction between
/dev/ttyACM* and /dev/ttyUSB* but it isn't now. Also, it's unlikely that
the same vid/pid pair will be shared between an acm and usb driver
anyway.
This can be very useful for some purposes:
* It makes it possible to disable the UART in cases where it is not
needed or needs to be disabled to conserve power.
* It makes it possible to disable the serial output to reduce code
size, which may be important for some chips. Sometimes, a few kB can
be saved this way.
* It makes it possible to override the default, for example you might
want to use an actual UART to debug the USB-CDC implementation.
It also lowers the dependency on having machine.Serial defined, which is
often not defined when targeting a chip. Eventually, we might want to
make it possible to write `-target=nrf52` or `-target=atmega328p` for
example to target the chip itself with no board specific assumptions.
The defaults don't change. I checked this by running `make smoketest`
before and after and comparing the results.
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.
This also fixes a bug: the Bluefruit doesn't have a low frequency
crystal. Somehow non-SoftDevice code still worked. However, the
SoftDevice won't initialize when this flag is set incorrectly.
Add a target for the Adafruit Circuit Playground Bluefruit, which is
based on the nRF52840. Adds the necessary code for the machine
package and the json and linker script files in the targets directory.
The machine package code is based on board_circuitplay_express.go,
with modifications made by consulting the wiring diagram on the
adafruit website here:
https://learn.adafruit.com/adafruit-circuit-playground-bluefruit/downloads
Also adds support to the uf2 conversion packacge to set the familyID
field. The Circuit Playground Bluefruit firmware rejects uf2 files
without the family id set to 0xADA52840 (and without the flag specifying
that the family id is present).
