Add 'tmp' to the list of FILESYSTEMS dependencies. Some scripts that
depend on FILESYSTEMS run mktemp(1). For systems that have read-only
root this is broken until memory disk based /tmp is instantiated. At
least 'os-release' and 'motd' are subject to this problem.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D28097