boards: use machine.Serial instead of machine.UART0

[aykevl]

See: tinygo-org/tinygo#1880

Untested, I'm not sure how to run the tests:

$ make all
rm -rf build
mkdir -p build
go build -o build/testrunner tools/testrunner/*
tinygo flash -size short -target=itsybitsy-m4 -port=/dev/itsybitsy_m4 ./itsybitsy-m4/
main module (tinygo.org/x/tinyhci) does not contain package tinygo.org/x/tinyhci/itsybitsy-m4
make: *** [Makefile:16: test-itsybitsy-m4] Error 1

[deadprogram]

It needs some tests to test itself, I'm afraid.

I tested on the physical test setup with one of the failing tests, and it passed, so merging. Thanks!