tools: kwboot: Handle EINTR in kwboot_write()

The write() syscall may be interrupted. Handle EINTR and retry it.

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