make: Implement optional features

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.
make: expose used features with `-DFEATURE_ABC`

All used feature requests (both in `FEATURES_REQUIRED` and
`FEATURES_OPTIONAL`) that could be satisfied get exposed to C with
`-DFEATURE_ABC`.
make: allow drivers to implement features

This change allows drivers (or any module for that matter) to provide
features. This is e.g. useful if a board does not have a transceiver,
but your application uses `USEMODULE += some_driver`, which implements
the transceiver interface.

The line `FEATURES_PROVIDED += some_feature` should go to the guarded
block in `{sys,drivers}/Makefile.include`.
Merge pull request #1879 from Kijewski/issue-1876

make: fix `FEATURES_REQUIRED` and `FEATURES_PROVIDED` a bit