This adds a `periph_sercom` feature and implementation which
`periph_i2c`, `periph_uart`, and `periph_spi` are implemented on top.
This allows for sharing a single SERCOM instance to provide multiple
serial interfaces (in round-robin time-sharing fashion).

Note: In practice, a SERCOM can often not be shared if it needs to
      provide an UART.

Background:

While code using the I2C/SPI APIs is already optimized to share the
peripheral with `i2c_acquire()`/`spi_acquire()` and
`i2c_release()`/`spi_release()`, UARTs are typically not shared and most
users will not call `uart_poweron()` and `uart_poweroff()` to only have
the SERCOM in UART mode when actually needed. Worse: For many use cases
(such as stdin), the UART will need to be constantly running, as
receiving data happens asynchronously at unpredictable points in time.

Now that sharing SERCOMs is implemented, we can make use of that :)

A warning is emitted if this is actually done, as sharing with UART is
often not working as expected.

Calling `uart_poweroff()` when done with the UART test allows sharing
the underlying hardware e.g. to provide other peripheral interfaces.
One example of this would be the SERCOM3 on the Adafruit Metro M4
Express that is used to provide UART on D1/D0 and SPI on D11/D12/D13.

We cannot use the D0/D1 UART if it is also used for STDIO. However,
the logic did not take into account whether `stdio_uart` was used at
all. This fixes the issue.

The M4 Express does now have an I2C and an UART compatible with Arduino
Shields, so we can expose them.

Also detect TTY of bootloader with `MOST_RECENT_PORT=1`.