Add support for generating a Windows only tools ISO

Commit 1: Adds make ISO step to the windows stage script. This will not break existing build machines that do not have the genisoimage for cygwing. Once aded the stage step will start creating the ISO. Sneak in a change to the volume name for the xc-tools ISO too.

Commit 2: This adds the genisoimage command. Also update the cygwin installer
checksum as there is a new one.

OXT-65

Signed-off-by: Ross Philipson <ross.philipson@gmail.com>