arm64: don't loop forever if first option in kern.cfg.order not available

strchr returns a pointer to the ',', so if the first option in the list
isn't available, we need to step over the , to look at the next
option. So if kern.cfg.order="acpi,fdt" and we have no acpi, we'd loop
forever with order=',fdt'.

Sponsored by: Netflix
Reviewed by: andrew, jhb
Differential Revision: https://reviews.freebsd.org/D36682