This feature is only used to blacklist stdio via CDC ACM.
Introduce the `highlevel_stdio` feature instead to indicate
that stdio functionality requires a running thread to print
anything
CDC ACM, ethos, Semihosting and SLIP all benefit from sending data out
in larger chunks and will benefit from stdout buffering.
`stdio_rtt` does have an internal TX buffer, so we don't need to do any
buffering on top.
With plain UART there was a slight advantage *without* buffering when
testing with `tests/periph_uart_nonblocking` (with the non-blocking feature
disabled).
Since the removal of the buffering saves us some RAM and ROM, disable it by
default there.
This will be different with DMA enabled UARTs.
Support for picolibc as alternative libc implementation is added with
this commit. For now only cortex-m CPU's are supported.
Enable via PICOLIBC=1
---
v2:
squash fixes in
v3:
Remove picolibc integer printf/scanf stuff from sys/Makefile.include,
it gets set in makefiles/libc/picolibc.mk
fixup for dependency
