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.
This change was introduced in 40adda8665bbcda1bad7e3a6a3c1031027faf39a, and partially fixed in 1ce07411fae8986f3acc293c9fb9ebee5211056a.