debian-installer now has extra steps to create a grub_embed file which
can be passed to xorriso's --embedded-boot option. Add corresponding
code in debian-cd to use it to make images USB bootable.
Patch from Samuel Thibault.
When making a custom server netinstall ISO, it may be desired to not
have MS Windows files. Unfortunately there's currently no check for
loadlin.exe, so install.bat gets created regardless. Fix this.
debian-cd currently expects there to always be isolinux/desktop/*
files and a dtmenu.cfg file, failing if they are missing. Make it so
debian-cd can cope whether those files are present or not.