generator: Deduplicate ostree= karg parsing

generator: Deduplicate ostree= karg parsing

Avoid having two copies of a regular expression for parsing
the `ostree=` kernel argument.  Because the `ostree-system-generator`
binary already has access to the internals because it's implemented
in the shared library, expose the sysroot version internally
and use that.

Motivated by an attempt to change one of these copies but not
the other.