nanobsd: Revert back to 1G (really 927M) partitions for rescue.

The last bump to 8G was caused by all the new symbols and debug info we
install by default. Turning those off gets us back to a reasonable size:

Filesystem Size Used Avail Capacity Mounted on
/dev/md1s1a 927M 771M 83M 90% /mnt

Sponsored by: Netflix
Reviewed by: jlduran@gmail.com
Differential Revision: https://reviews.freebsd.org/D43232