tools: kwboot: Refactor and fix writing buffer

tools: kwboot: Refactor and fix writing buffer

There are 3 instances in kwboot.c where we need to write() a given
buffer whole (iteratively writing until all data are written), and 2 of
those instances are wrong, for they do not increment the buffer pointer.

Refactor the code into a new function kwboot_write() where it is fixed.

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