image: Add partition file-system options support.

* gnu/image.scm (<partition>)[file-system-options]: New field,
(partition-file-system-options): new exported procedure.
* gnu/system/image.scm (partition->gexp): Adapt accordingly.
* gnu/build/image.scm (sexp->partition): Also adapt accordingly,
(make-ext-image): and pass file-system options to mke2fs.