machine: define Serial as the default output

Previously, the machine.UART0 object had two meanings:

  - it was the first UART on the chip
  - it was the default output for println

These two meanings conflict, and resulted in workarounds like:

  - Defining UART0 to refer to the USB-CDC interface (atsamd21,
    atsamd51, nrf52840), even though that clearly isn't an UART.
  - Defining NRF_UART0 to avoid a conflict with UART0 (which was
    redefined as a USB-CDC interface).
  - Defining aliases like UART0 = UART1, which refer to the same
    hardware peripheral (stm32).

This commit changes this to use a new machine.Serial object for the
default serial port. It might refer to the first or second UART
depending on the board, or even to the USB-CDC interface. Also, UART0
now really refers to the first UART on the chip, no longer to a USB-CDC
interface.

The changes in the runtime package are all just search+replace. The
changes in the machine package are a mixture of search+replace and
manual modifications.

This commit does not affect binary size, in fact it doesn't affect the
resulting binary at all.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/machine/board_pyportal.go b/src/machine/board_pyportal.go
index ff3ad1a1a..c2ffb386e 100644
--- a/src/machine/board_pyportal.go
+++ b/src/machine/board_pyportal.go
@@ -94,7 +94,9 @@ const (
 	LED = D13
 )
 
-// UART0 aka USBCDC pins
+var Serial = USB
+
+// USBCDC pins
 const (
 	USBCDC_DM_PIN = PA24
 	USBCDC_DP_PIN = PA25