Check if the extracted image will fit onto the designated partitions

[mmstick]

It's possible for the install process to fail due to running out of space on a partition, if the partition is smaller than what the image requires. The simplest way to go about this is to ensure that the root partition is at least 3x the size of the image (maybe we should include a file that shows the actual file size of the compressed image?). More complex checking may not be required, but would be welcomed (ie: recursively checking if the contents of the image will fit onto each partition that will be mounted).