rc: Allow the removal of firstboot_sentinel on read-only file systems

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 40adda8665bb, and partially fixed in
1ce07411fae8.

Differential Revision: https://reviews.freebsd.org/D34166