Enhancement: Add multi-partition support on flash drives

[Howard Chu]

These days a typical flash drive has much larger capacity than a typical OS iso image. It's a shame to waste all the extra space just to load a single OS image. It would be nice to add an option to copy the OS image to a specific partition of the flash drive instead. I've been doing this manually so far, but would prefer a simple UI for it. Aside from installing the bootsector, you also need to toggle the boot flag on the desired partition. This also allows a single flash drive to be used to switch between multiple OS images, just by setting the boot flag on the particular partition.