uart0 functionality is removed by #3164. This patch implements periph/uart,
rather than deprecated uart0, using /dev/tty.
To use with netdev2_tap simultaneously, this patch adds asynchronus read system
and modifies netdev2_tap to use it.
A TTY device is specified on command line with -c (COM) option, since -t was
used by the old implementation.
This patch also implements empty GPIO driver needed by the xbee driver.
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`.
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.
