makefile/..picolibc: make missing picolibc fail louder

[kfessel]

Contribution description

missing picolibc auto adaptation needs changes in make-system and is therefor on hold this PR make the build fail early and loud and needs basically no modification of unrelated make-system parts.

Testing procedure

Build something that prefers or requires picolibc on a system that does not have it (installed) (e.g.: current Ubuntu LTS 20.04)
e.g.: RIOT/examples/gnrc_minimal for a cortex-m Board

before u get a failure somthing like:

"make" -C <Riot>/sys/picolibc_syscalls_default
<Riot>/sys/picolibc_syscalls_default/syscalls.c:236:5: error: implicit declaration of function 'FDEV_SETUP_STREAM' [-Werror=implicit-function-declaration]
  236 |     FDEV_SETUP_STREAM(picolibc_put, picolibc_get, picolibc_flush, _FDEV_SETUP_RW);
      |     ^~~~~~~~~~~~~~~~~
<Riot>/sys/picolibc_syscalls_default/syscalls.c:236:67: error: '_FDEV_SETUP_RW' undeclared here (not in a function)
  236 |     FDEV_SETUP_STREAM(picolibc_put, picolibc_get, picolibc_flush, _FDEV_SETUP_RW);
      |                                                                   ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors

somewhere in the build process

after the patch you get:

<Riot>/makefiles/libc/picolibc.mk:10: picolib was selected to be build but no picolib.spec could be found
<Riot>/makefiles/libc/picolibc.mk:11: *** check your installation or build configuration.. Stop.

right at the start

Issues/PRs references

PR.: to fix the selection process (need make-system reordering): #15993
Issue about failed selection process: #15325
Issue about the criticality of order of inclusion: #9913

@maribu, @fjmolinas might want to have a look at this

[maribu]

Maybe a more concrete help message? In the end, it comes down to these choices:

1. install picolibc for $(TARGET_ARCH)
2. add FEATURES_REQUIRED += newlib to the Makefile
3. Run export FEATURES_REQUIRED=newlibc

[kfessel]

I am all in for improvements to the wording of the message. The current message is short and technical but maybe less helpful.

But the preference may have been done at so many places that i wasn't sure how to word an instant help like remove 'Feature_Optional += picolib ' from every module in use or Force newlib by using Feature Required

adding FEATURES_REQUIRED += newlib to the Makefile disables native build

installing picolibc is impossible for Ubuntu LTS (20.04) (the reason why i had to dist-upgrade the other day since i wanted to check my other PR)

adding FEATURES_REQUIRED += newlib to the Makefile/export still does not work for the gnrc_minimal example

but you get an really beautyfull warning message from #15973 (not realy but ii dont know better):

The following features may conflict: newlib picolibc
Rationale: Only one standard C library can be used

EXPECT undesired behaviour!

[maribu]

An alternative approach would be to go for FEATURES_BLACKLIST += picolibc instead. But I think the build system shouldn't consider optional features that would end up in a conflict for usage. A fix is PR'ed here: #16012

[kfessel]

i knew i saw some PR by you regarding that dependency issue

[kfessel]

I will add that suggestion to the PR

[kfessel]

added sugestions and rebased on current master to make them work as expected.

current warning words are:

<Riot>/makefiles/libc/picolibc.mk:10: picolibc was selected to be build but no picolibc.spec could be found
<Riot>/makefiles/libc/picolibc.mk:11: you might want to install "picolibc" for "arm-none-eabi"
<Riot>/makefiles/libc/picolibc.mk:12: or add "FEATURES_BLACKLIST += picolibc" to Makefile
<Riot>/makefiles/libc/picolibc.mk:13: *** check your installation or build configuration.. Stop.

i think this is ready for ci

[kfessel]

I force pushed some "c"s to the picolibc