cpu/atmega_common: refactor AVR libc code into module

[kaspar030]

This PR refactors atmega libc specific code and compatibility headers into a
subfolder.

Test if CI still builds, sizes stay same.

[kaspar030]

• rebased

@kYc0o care to take a look at this?

[kaspar030]

here is the new static-tests output.

[smlng]

ACK, tested on Arduino-Mega2560. Confirm it still works and size(s) stay the same! Murdock error is (very) likely unrelated.

[smlng]

can someone else please verify this, too: @MrKevinWeiss @leandrolanzieri? Just to be sure

[smlng]

@kaspar030 please squash if Murdock is happy

[MrKevinWeiss]

running tests now

[MrKevinWeiss]

Both master and this PR fail the same tests using @cladmi compile_and_test_for_board.py script

Failures during test:
- [tests/bench_runtime_coreapis](tests/bench_runtime_coreapis/test.failed)
- [tests/bitarithm_timings](tests/bitarithm_timings/test.failed)
- [tests/evtimer_msg](tests/evtimer_msg/test.failed)
- [tests/isr_yield_higher](tests/isr_yield_higher/test.failed)
- [tests/libfixmath](tests/libfixmath/test.failed)
- [tests/periph_eeprom](tests/periph_eeprom/test.failed)
- [tests/periph_gpio](tests/periph_gpio/test.failed)
- [tests/pipe](tests/pipe/test.failed)
- [tests/pkg_fatfs_vfs](tests/pkg_fatfs_vfs/test.failed)
- [tests/pkg_jsmn](tests/pkg_jsmn/test.failed)
- [tests/pkg_libb2](tests/pkg_libb2/test.failed)
- [tests/pkg_lora-serialization](tests/pkg_lora-serialization/test.failed)
- [tests/pkg_micro-ecc](tests/pkg_micro-ecc/test.failed)
- [tests/pkg_tiny-asn1](tests/pkg_tiny-asn1/test.failed)
- [tests/posix_semaphore](tests/posix_semaphore/test.failed)
- [tests/ps_schedstatistics](tests/ps_schedstatistics/test.failed)
- [tests/rng](tests/rng/test.failed)
- [tests/trickle](tests/trickle/test.failed)

[kYc0o]

Maybe this can come after #9130 ?

[smlng]

Maybe this can come after #9130 ?

I think the order is not important, its more a matter of who needs to rebase (and maybe manual merge stuff) later on 😄

[kaspar030]

• squashed

Can this be considered minor impact regarding the freeze?

[smlng]

reACK, tested with arduino-mega2560 and several different test applications: all good. Also tested with Arduino-uno, same same. GO!

[miri64]

Am I right to understand that this PR reverts parts of #8857 (specifically b597700)?

[kaspar030]

Am I right to understand that this PR reverts parts of #8857 (specifically b597700)?

Yes and no. #8857 updated vendor files and moved them from avr-libc-extra/include into include/vendor in order to keep CI from checking them. This PR re-introduces avr-libc-extra/ but also adds a vendor/ subfolder.

Having avr-libc-extra doesn't make sense as is (if all AVR boards always use the avr libc), but is a first step towards either not using a libc at all or using a different one.