Older versions of newlib already provide the magic endian numbers
via `machine/endian.h`, which may be indirectly included. This changes
the header to only provide the macros if the are not provided otherwise.
For sanity, it checks if the values are indeed the expected magic
numbers, even if provided from other sources.
The constants BIG_ENDIAN etc. were not consistently defined. This
bug was not caught by the unit tests, as the preprocessor would
treat undefined macros as being `0` in numeric comparisons, hence
the following test did select the correct implementation:
```C
#if BYTE_ORDER == LITTLE_ENDIAN
/* correct implementation for all currently supported boards */
#endif
```
This adds now an explicit test to the unit tests to ensure that the
magic numbers are consistently the correct values. Hence, this bug
should no longer be able to sneak in.
Co-authored-by: Teufelchen <9516484+Teufelchen1@users.noreply.github.com>
This provides glibc, NetBSD, FreeBSD compatible endian.h header with a
lean and simple API to convert between host byte order to little endian
and big endian and the other way around.
