bootadm: add bootfile fallback to unix
reboot command can pass temporary (transient) flags and other data to bootadm, to create one time boot setup. The loader does read kernel name from bootfile environment variable, and if the value is messed up, we fail to boot. Of course this can be fixed manually by: set bootfile=unix but we should provide the fallback value so we wont fail the boot.
This with 8986 will fix the "interesting" error cases like one from entering: reboot now