Bootable USB disk that lets you choose an ISO image

This is basically a newer iteration on https://mg.pov.lt/blog/booting-iso-from-usb.html

Creating a bootable USB disk that lets you boot any Ubuntu ISO image:

#. Mount a USB disk with a sufficient amount of free space. Note the device name (e.g. /dev/sdb) and the mount point (e.g. /media/mg/MG-FLASH).

#. Install GRUB:

```
sudo grub-install --root-directory=/media/mg/MG-FLASH /dev/sdb
```

(you may have to also use --force)

#. Perhaps also install an UEFI bootloader

```
sudo grub-install --target=x86_64-efi --removable \
                  --root-directory=/media/mg/MG-FLASH \
                  --efi-directory=/media/mg/MG-FLASH /dev/sdb
```

(this is totally untested)

#. Download Ubuntu ISO images you want

```
cd /media/mg/MG-FLASH
git clone https://github.com/mgedmin/ubuntu-images ubuntu
cd ubuntu
make verify-all
```

#. Check out this repository (this is tricky because git doesn't want to check out things into an existing non-empty directory)

```
git clone https://github.com/mgedmin/bootable-iso /tmp/bootable-iso
mv /tmp/bootable-iso/.git /media/mg/MG-FLASH/boot/grub/
mv /tmp/bootable-iso/* /media/mg/MG-FLASH/boot/grub/
```

#. Edit /media/mg/MG-FLASH/boot/grub/grub.cfg so it matches your Ubuntu images

#. Test that things work

Testing with KVM

#. Unmount the device

```
udisksctl unmount -b /dev/sdb1
```

#. Boot it in KVM

```
sudo setfacl -m user:$USER:rw /dev/sdb
kvm -m 2048 -k en-us -drive format=raw,file=/dev/sdb
```

#. When you're done testing, mount the device again with

```
udisksctl mount -b /dev/sdb1
```

Adding new boot menu entries

1. Edit grub.cfg.
2. Copy an existing menu/submenu that is similar.
3. Change version numbers.
4. Launch mc (Midnight Commander), find the ISO image, press Enter to look inside.
5. Locate the boot/grub/grub.cfg file inside the ISO image.
6. Copy the kernel command-line arguments exactly.
7. For desktop ISO images add iso-scan/filename=$isofile on the kernel command line, before -- or ---. For some reason server ISO images don't need this (at least the ones using debian-installer, i.e. before 18.04 LTS).