cache device: take over unpartitioned space

Now that we can use boot2container on single board computers, we probably want to make it easy to use it there!

Single board computers usually require the bootloader and the OS to be on the same boot media (SD card, eMMC, USB stick, ...), so the current automatic partitioning will end up erasing the SD card... thus preventing future boots from happening :D

I would thus recommend we ship basic bootloaders that could be dd'ed onto an sdcard (regardless of its size), then at the first boot, b2c would add a new partition in the empty space and use that as the cache storage.