tools: kwboot: Handle EINTR in kwboot_tty_recv()

The select() and read() syscalls may be interrupted. Handle EINTR and
retry them.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>