Move prepare-root karg helpers into otcore, add unit tests

Move prepare-root karg helpers into otcore, add unit tests

Add long overdue unit testing coverage for this, which
at least slightly closes out the android boot CI gap.

Actually, this *copies* the karg parsing code into otcore because
it now uses glib, which we're not yet using in the static
prepare-root.  It's pretty tempting to drop support for the
static prepare root entirely.  But for now we'll live with some
code duplication.