sys/stdio_uart: add pseudomodule stdio_flush_rx

[HendrikVE]

Contribution description

This PR introduces a new pseudomodule. It is currently only implemented for stdio_uart.

The use case is to clear the input buffer so we can throw away the remaining input (like \n for example) after a call of scanf for the next call of scanf.

Testing procedure

Have an application with multiple calls of scanf and use stdio_flush_rx at some places and don't use it on others.

[kaspar030]

bikeshedding: you'd "flush" output, "drain" input?

[HendrikVE]

bikeshedding: you'd "flush" output, "drain" input?

I think I'd rather go with a simple "clear" then instead, maybe "stdio_clear_input"?

[MrKevinWeiss]

Should we get this in for the release?

[riot-ci]

Murdock results

:x: FAILED

8d4c2e9eece08a3ac386fa171312afebfcdd75a1 sys/stdio_uart: add pseudomodule stdio_flush_rx

Success	Failures	Total	Runtime
152	0	7956	01m:37s

Artifacts

[benpicco]

Needs a rebase, you can move this to sys/stdio/stdio.c now.

[fabian18]

What about ethos? stdio.c is not being compiled when using stdio_ethos.

stdio.inc.mk

STDIO_LEGACY_MODULES = \
  ethos_stdio \
  stdio_ethos \
  stdio_native  # requires #19002 \
  #

# select stdio_uart if no other stdio module is slected
ifeq (,$(filter $(STDIO_MODULES),$(USEMODULE)))
  USEMODULE += stdio_uart
endif

ifeq (,$(filter $(STDIO_LEGACY_MODULES),$(USEMODULE)))
  USEMODULE += stdio
endif

undefined reference to 'stdio_flush_rx' collect2: error: ld returned 1 exit status

[benpicco]

True, ethos would need a custom implementation. Alternatively it could be ported to the new interface which would require something like https://github.com/RIOT-OS/RIOT/pull/18066