Fixes#1876
This PR introduces `FEATURES_OPTIONAL` which can be used to tell the
Make system, that the application would like to use some feature, but
the build should proceed even if the selected board cannot provide the
optional feature.
`make buildtest` and `make info-supported-boards` heed this variable
when examining the list of supported boards.
If a word is present in `FEATURES_REQUIRED` and `FEATURES_OPTIONAL`,
then `FEATURES_OPTIONAL` takes precedence.
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.
Currently, the tcp and udp implementations are bound to each other in a
module called *destiny*. Thus, when using only one of them then the
other one gets also compiled into the binary and initialized,
which results in unnecessary RAM usage and workload for the CPU.
The approach in this PR defines a common module named *socket_base*,
which contains functions used by the posix layer. Compiled by it's own,
those functions return negative error codes, to symbolize upper layers
that they are not supported. When also including the modules *udp* or
*tcp* respectively, functions from *socket_base* get overwritten with the
correct functionality.
Defining *udp* or *tcp* in a Makefile also includes *socket_base*.
Defining *pnet* in a Makefile also includes *socket_base*.
