treewide: model newlib as a FEATURE

[fjmolinas]

Contribution description

This PR models newlib as a feature and makes use of FEATURES_REQUIRED_ANY to allow requesting any of both picolibc and newlibc while only having one selected.

This way applications can enforce one of them with FEATURES_REQUIRED += %. IMO only applications should enforce this, otherwise, a conflict is likely to arise, FEATURES_CONFLICT is added to prevent this.

Testing procedure

Verify that dependencies are still correct. Some here

→ BOARD=samr21-xpro make -C examples/gnrc_minimal/ info-modules | grep lib
picolibc
picolibc_syscalls_default

→ BOARD=hifive1b make -C examples/gnrc_minimal/ info-modules | grep lib
picolibc
picolibc_syscalls_default

→ RIOT_CI_BUILD=1 BOARD=hifive1b make -C examples/gnrc_minimal/ info-modules | grep lib
newlib
newlib_nano
newlib_syscalls_fe310

→ BOARD=nucleo-l011k4 make -C examples/hello-world/ info-modules | grep lib
picolibc
picolibc_syscalls_default

→ BOARD=nucleo-l011k4 make -C examples/hello-world/ info-modules | grep lib
picolibc
picolibc_syscalls_default

→ BOARD=samd10-xmini make -C examples/hello-world/ info-modules | grep lib
picolibc
picolibc_syscalls_default

→ BOARD=stm32f030f4-demo make -C examples/hello-world/ info-modules | grep lib
picolibc
picolibc_syscalls_default

FEATURES_REQUIRED=newlib BOARD=hifive1b make -C examples/hello-world/ info-modules | grep lib
newlib
newlib_nano
newlib_syscalls_fe310

BOARD=esp32-wroom-32 make -C examples/gnrc_minimal/ info-modules | grep lib
newlib
newlib_nano
newlib_syscalls_default

Issues/PRs references

Split from #15832

[fjmolinas]

@aabadie if you don't find a typo or other such change, mind triggering the ci?

[fjmolinas]

Forgot to add the symbol

[fjmolinas]

All green @aabadie!

[aabadie]

Very nice and required cleanup ! Thanks @fjmolinas.

I trust your testing and Murdock is all green.

ACK!

[fjmolinas]

Thanks for the review!