push_image.sh: support intel/desktop

[Christian Stewart]

Intel/desktop has a different file layout from usual:

boot/
├── bzImage-skiffos-2022.11.1-50-g107de564-1
├── init-skiffos-2022.11.1-50-g107de564-1.squashfs
├── refind_linux.conf
└── skiff-init
    └── skiff-init-squashfs

To fully support this:

• push_image.sh: correctly mounts /mnt/boot with --rbind
• push_image.sh: needs to correctly add the version suffix to bzImage
• push_image.sh: needs to correctly name the rootfs.squashfs file

[Christian Stewart]

Added a script under scripts/ specifically for intel/desktop, considering this done for now, could be improved / merged with push_image.sh in future.