tools: kwboot: Disable non-blocking mode

The kwboot utility does not handle EAGAIN / EBUSY errors, it expects
blocking mode on tty - it uses select() to check if data is available.

Disable non-blocking mode by clearing O_NDELAY flag which was set by
open().

We can't just take O_NDELAY from open(), because it is required there
until the CLOCAL flag is set on the tty.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>