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>
