Extract ZFS related rules from main makefile to zfs module

[wkozaczuk]

With d7a6274, 9cc49b3 and other recent ZFS related commits, ZFS support is now part of a standalone library libsolaris.so that can be optionally added to the image. We even have a module zfs that in essence adds this library if building images where libsolaris.so is loaded from the library.
Given this, it would be nice to move all ZFS craft from the main makefile to the separate one under the zfs module.

[dhruvsethi227]

Hello, my name is Dhruv and I am a senior computer science student at UT Austin. My group and I were wondering if we could tackle this issue as part of our virtualization class assignment.

[wkozaczuk]

That should be pretty easy. Go for it.