include: add SPDX GPL-2.0-only license tags for files without licensing information ...

According to our /README, GPL-2.0-only applies for the whole project
except noted otherwise.

Signed-off-by: Roland Hieber <rohieb@rohieb.name>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

serdev: Do not call .receive_buf() callback recursively ...

Code implementing .receive_buf() callback can potentially call
serdev_device_write(), which will call serdev_device_poller(). We need
to make sure that such a call is a no-op in order to prevent
corrupting shared data buffer as well as breaking .receive_buf
callback that most likely does not expect that to happen.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Tested-by: Cory Tusar <cory.tusar@zii.aero>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

serdev: Add trivial blocking read function ...

Add example implementation of a trivial blocking read function as a
part of Barebox's serdev API.

NOTE: This code has not been tested against real devices, so use it at
your own risk.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

console: Add simplified 'serdev' framework from Linux kernel ...

Port 'serdev' UART-slave deivce framework found in recent Linux
kernels (post 4.13) in order to be able to port 'serdev' slave drivers
from Linux.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>