NanoBSD or, more generally, systems with `root_rw_mount="NO"` are not able to remove the `firstboot_sentinel` file, typically `/firstboot`, because the logic in `/etc/rc` is currently inverted.
When `checkyesno root_rw_mount` tests on a read-only file system, the return is `1`, hence avoiding the option to mount the system read-write.
Restore the ability to remove the `firstboot_sentinel` file on read-only mounted file systems, and allow said systems to `growfs(8)` upon first boot.
This change was introduced in 40adda8665bbcda1bad7e3a6a3c1031027faf39a, and partially fixed in 1ce07411fae8986f3acc293c9fb9ebee5211056a.