Set MAKE_ISO=yes so ubuntu-mini-iso uses the standard isobuilder
flow in auto/build. The binary hook is simplified to just creating
kernel/initrd artifacts; isobuilder handles .disk metadata, boot
configuration, and ISO creation.
The mini-iso's custom grub.cfg (single iso-chooser-menu entry) is
generated by a project-specific path in AMD64BootConfigurator.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>