From the matrix: "stdio in RIOT is a hot mess". So let's fix this. This lists the tasks needed for sane stdio

• Fix CONFIG_SKIP_BOOT_MSG makes newlib stdio races more likely #20067
  • Maybe just explicitly initializing the internal stdio structures of newlib prior to going to multi-threading mode?
• Agree one semantics of stdio (see sys/stdio: Precision on semantics, and fixes #20364)
  • Best effort?
  • Blocking? Non-blocking?
  • Should lost characters be reported? To whom?
• Fix stdio backend drivers to provide consistent semantics
  • stdio_uart
  • stdio_semihosting
  • stdio_rtt
  • stdio_cdc_acm
  • stdio_nimble
  • stdio_udp
  • stdio_telnet
  • stdio_usb_serial_jtag
  • stdio_native
  • stdio_ethos
  • slipdev_stdio