sys/stdio_ethos: replace USE_ETHOS_FOR_STDIO by stdio_ethos pseudomodule

[aabadie]

Contribution description

This PR provides a new module stdio_ethos as a replacement for USE_ETHOS_FOR_STDIO. Applications using this macro now depends on this new module.

stdio_ethos also depends on the ethos and stdin, provided in #11598.

Testing procedure

• examples/gnrc_border_router should still work
• related tests application should still pass when using stdio_ethos: tests/riotboot_flashwrite, tests/gnrc_ipv6_ext, tests/gnrc_rpl_srh and tests/gnrc_sock_dns.

Issues/PRs references

Based on #11598

[cladmi]

It would be nice to have it implemented without #11598 as this one could be done by adding stdio_uart_rx dependency alone.

Some minor remarks inline.

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Further steps for other pull requests.

From the implementation, it looks like stdio_ethos should be moved to its own module like stdio_rtt. But it is another task that requires many changes.

Also the code is doing implicit circular dependency, the code in drivers/ethos for stdio references variables defined in stdio_uart…

drivers/ethos/ethos.c:extern isrpipe_t stdio_uart_isrpipe;

[aabadie]

It would be nice to have it implemented without #11598 as this one could be done by adding stdio_uart_rx dependency alone.

Once #11598 is merged (and I hope it will be soon), there will be no need to rework this one. Let's finish #11598, which will fix #11525 btw, and continue improving other things afterwards.

[kaspar030]

Almost good to go. Please add the missing "stdio_uart" dependency.

[kaspar030]

please rebase

[kaspar030]

Please squash!

[aabadie]

squashed!

[kaspar030]

ACK.

I found another reference of USE_ETHOS_STDIO in ethos' own README.md and pushed a commit that updates it.
Please take a quick look.
Otherwise I tested that gnrc_border_router still works using make term.

Comments have been addressed, reviewer on holidays. Thx @cladmi!

[aabadie]

Please take a quick look.

Looks good. Thanks!