f2fs: Update make rules for CM compatibility

* Make sure fsck.f2fs and mkfs.f2fs are built for CM12.1/13.0
* Add CM13.0 rule for relinking mkfs.f2fs
* Minor cleanup of nested if/else in prebuilts

Note on CM SDKs: 3=cm-12.1, 4=cm-13.0
I'm not yet sure whether it is possible that the CMSDK increments
within the same branch of CM.

Change-Id: I7b521159a8b5d0e0ea639f5e0fdd885100612df0