pkg/cache_device: take over unpartitioned space if possible

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

Additionally, this feature could be used to speed up boot time of PXE-netbooted machines, as it would allow flashing iPXE to the main drive of the machine and thus preventing the need for the firmware to first acquire an IP address then download iPXE (~5s) only for iPXE to re-do the exact same thing.

Closes: #6 (closed)