nanobsd: Better NANO_OBJ if NANO_LAYOUT is set

If NANO_LAYOUT is set, then use /usr/obj/nanobsd.${NANO_NAME}.${NANO_LAYOUT}
instead of the current /usr/obj/nanobsd.${NANO_NAME} to allow multiple layouts
to be built w/o errors due to the time-skew that creates.

PR: 269366
Suggested by: Eugene Grosbein
Sponsored by: Netflix