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.
The SoftDevice should already be installed on these chips. Adding the
right build tags makes them work with the bluetooth package.
I did not change the HasLowFrequencyCrystal property: all these boards
use the MDBT50Q which appears to include a low-frequency oscillator.
That is, I tested the ItsyBitsy nRF52840 with the property set to true
and advertisement worked just fine.
