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 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).