allow choosing compression algorithm for mksquashfs

Current with newer kernels uck defaults to xz compression for mksquashfs, which results in the most tightly packed ISO.

But this does not perform very well on older systems or lower end newer systems (think Atom netbooks).

Keeping below the 700MB is harder and harder to do these days, so best possible compression is becoming less important.

With LZO the image becomes about 30% larger in my case (when compared to xz), but on an Atom netbook the system boot significantly quicker and after boot applications starter faster too.

So it would be nice to be able to choose the compression type.