Please see #1715.
Closes#1715.
This PR implements the new Makefile variables "FEATURES_PROVIDED" and
"FEATURES_REQUIRED". A board *can* have a new file `Makefile.features`
which looks like:
```make
FEATURES_PROVIDED = transceiver
```
An application can have a corresponding line
```make
FEATURES_REQUIRED = transceiver
```
If the selected BOARD does not fulfil the requirements of the
application, then a *warning* is issued at compile time.
This change only includes the feature "transceiver", further features
are expected to be listed in further PRs. The requirement "transceiver"
is automatically added if the application uses the module
"defaulttransceiver".
`make buildtest` understands the new feature listing, so the user won't
need to add boards to `BOARD_BLACKLIST` manually.
Part of the change are the added Make targets
* `info-features-missing`, which prints the required features
`\setminus` the provided features. The output is empty if there are no
features missing.
* `info-boards-features-missing`, the same as `info-features-missing`
but as a table for all boards, but heeded `BOARD_WHITELIST` and
`BOARD_BLACKLIST`.
Applications don't have to use this new feature. This change does not
break existing Makefile.
`bloom_t` is defined as a struct.
`_t` can mislead the user to think of bloom_t
as a typedef (see our coding conventions) instead of a struct.
Thus, I modified `struct bloom_t` to be a *typedefed* struct.
Another solution would be to rename bloom_t to sth. like bloom_s
everywhere and use `struct bloom_s` instead of `bloom_t`.
Changing between branches in git typically leaves directories empty and
does not remove them. If you have different unittests in different
branches this leads to an error in building them, since the current
configuration expects a Makefile in all `tests-*` directory. This change
configures Make to search for a Makefile in `tests-*` rather than just
assuming that there is one.
