Contribution description

This PR provides the support for using the Serial over stdio_*

On Arduino boards, the Serial object uses

• either an UART interface connected to an USB interface via an additional ATmega chip (class HardwareSerial)
• or directly the USB CDC interface (class _Serial).

However, the Serial object in RIOT always uses the UART interface defined by ARDUINO_UART_DEV, usually UART_DEV(0). Therefore Serial does not work for boards like feather-m0 and arduino-mkr which use the USB CDC interface directly. Since the STDIO already uses the USB CDC interface for such boards with the help of the module stdio_cdc_acm, it is possible to realize the Serial object for such boards on top of the STDIO.

For this purpose the module arduino_serial_stdio is introduced. If this module is enabled AND the ARDUINO_UART_DEV is defined as UART_UNDEF, the STDIO is used for Serial. This allows either the use of the STDIO or the use of a UART interface if defined by ARDUINO_UART_DEV.

The module arduino_serial_stdio is automatically enabled for boards using stdio_cdc_acm. However, to actually use Serial via STDIO, ARDUINO_UART_DEV have to be defined additionally UART_UNDEF. This is done by this PR for

• feather-m0
• arduino-mkr.

For sodaq boards the situation is unclear, since ARDUINO_UART_DEV is explicitly defined for these boards and it is not clear whether this is intentional.

The use of Serial over STDIO requires that the used stdio backend implements the stdio_available function that was introduced by PR #17446.

Testing procedure

1. Use either a feather-m0 or an arduino-mkr board and use the following example to test:

   make BOARD=arduino-mkr1000 -C examples/arduino_hello-world flash term
   make BOARD=feather-m0 -C examples/arduino_hello-world flash term
   

   Enter a string should be echoed as expected.
2. Use any other board that uses std_uart, for example esp32-wroom and do the same:

   CFLAGS='-DARDUINO_UART_DEV=UART_UNDEF' USEMODULE=arduino_serial_stdio \
   make BOARD=esp32-wroom-32 -C examples/arduino_hello-world flash term
   

   Enter a string should be echoed as expected.

Issues/PRs references

Depends on PR #17446.