Add support for generating compressed depthcharge images

Some Chromebooks (google-kukui) have small kernel partition sizes (32M). While in the past this wasn't a problem, recent initramfs changes increased the size of the depthcharge image to be just above 32M (around 34M). One solution was to use the minimal initramfs on kukui, but this is less than ideal, as it only brings the depthcharge image size down to 31.9M, so there is very little room to grow in the future. A better solution is to compress the kernel itself, which brings the depthcharge image size down to ~12M. I tested compressed images on google-veyron, but they failed to work, so we can't just compress on every chromebook. This is why I created a deviceinfo variable to specify a compression type (lz4 and lzma are the only types supported). To maintain compatibility with the existing chromebook device packages, if a compression type isn't specified in deviceinfo, it will default to none.