rc: persistent firmware cache dirs on MFS; closes #924

This adds the following directories to avoid problems with
smaller embedded systems that do not have enough RAM:

	/var/cache/opnsense-update
	/var/cache/pkg

This is particularly bad for Nano systems with enough RAM
as it may grind the flash storage, but since this is a
"luxury" problem with Nano the scope is limited to that.
It's either being able to upgrade the firmware for all or
not at all.

FWIW, firmware upgrades will have to move to persistent
storage as soon as we hit FreeBSD 11 for the simple fact
that we'll have to do partial upgrade stages for the ABI
jumps of the kernel / base system anyway and cannot expect
to fetch more packages after reboot.

Proper garbage collection exists in both tools that shall
avoid overloading the persistant storage.

src/etc/rc

 #!/bin/sh
 
-# Copyright (c) 2014-2015 Franco Fichtner <franco@opnsense.org>
+# Copyright (c) 2014-2016 Franco Fichtner <franco@opnsense.org>
 # Copyright (c) 2004-2010 Scott Ullrich <sullrich@gmail.com>
 # Copyright (c) 2003-2004 Manuel Kasper <mk@neon1.net>
 # All rights reserved.
@@ -91,51 +91,78 @@ fi
 # Enable console output if its muted.
 /sbin/conscontrol mute off >/dev/null
 
-USE_MFS_TMPVAR=`/usr/bin/grep -c use_mfs_tmpvar /conf/config.xml`
-if [ ${USE_MFS_TMPVAR} -ne 0 ]; then
-	echo -n "Setting up memory disks..."
+setup_mfs_link()
+{
+	ROOT=${1}
+	MFS=${2}
+	NAME=${3}
+
+	# Create dummy directory to for MFS-bound
+	# directories that require a persistant
+	# storage underneath to run.
 
-	if [ ! -d /root/var/db/pkg ]; then
-		mkdir -p /root/var/db
-		mv /var/db/pkg /root/var/db
+	if [ ! -d "${ROOT}${MFS}/${NAME}" ]; then
+		mkdir -p "${ROOT}/${MFS}"
+		mv "${MFS}/${NAME}" "${ROOT}/${MFS}"
 		# create a symlink underneath as well
-		# to fix early boot pkg(8) issues:
-		ln -s /root/var/db/pkg /var/db/pkg
+		ln -s "${ROOT}${MFS}/${NAME}" "${MFS}/${NAME}"
 	fi
-
-	if [ ! -d /root/var/crash ]; then
-		mkdir -p /root/var
-		mv /var/crash /root/var
-		ln -s /root/var/crash /var/crash
+}
+
+install_mfs_link()
+{
+	ROOT=${1}
+	MFS=${2}
+	NAME=${3}
+
+	# Redirect persistent, but MFS-bound
+	# directory after tmpfs mount.
+
+	mkdir -p "${MFS}/${NAME}"
+	ln -s "${ROOT}${MFS}/${NAME}" "${MFS}"
+}
+
+delete_mfs_link()
+{
+	ROOT=${1}
+	MFS=${2}
+	NAME=${3}
+
+	# Persistent copies of MFS-bound directories
+	# still there must be moved back into place.
+
+	if [ -d "${ROOT}${MFS}/${NAME}" ]; then
+		mkdir -p "${MFS}"
+		# reverse the recovery symlink before
+		# moving back the original database
+		rm -f "${MFS}/${NAME}"
+		mv "${ROOT}${MFS}/${NAME}" "${MFS}/"
 	fi
+}
+
+USE_MFS_TMPVAR=`/usr/bin/grep -c use_mfs_tmpvar /conf/config.xml`
+if [ ${USE_MFS_TMPVAR} -ne 0 ]; then
+	echo -n "Setting up memory disks..."
+
+	setup_mfs_link /root /var/cache opnsense-update
+	setup_mfs_link /root /var/cache pkg
+	setup_mfs_link /root /var/db pkg
+	setup_mfs_link /root /var crash
 
 	mount -t tmpfs -o mode=01777 tmpfs /tmp
 	mount -t tmpfs tmpfs /var
 
-	mkdir -p /var/db
-	ln -s /root/var/db/pkg /var/db/pkg
-
-	mkdir -p /var
-	ln -s /root/var/crash /var/crash
+	install_mfs_link /root /var/cache opnsense-update
+	install_mfs_link /root /var/cache pkg
+	install_mfs_link /root /var/db pkg
+	install_mfs_link /root /var crash
 
 	echo "done."
 else
-	# User must have just disabled RAM disks,
-	# let's move these back into place.
-
-	if [ -d /root/var/db/pkg ]; then
-		mkdir -p /var/db
-		# reverse the recovery symlink before moving
-		# back the original database:
-		rm -f /var/db/pkg
-		mv /root/var/db/pkg /var/db/
-	fi
-
-	if [ -d /root/var/crash ]; then
-		mkdir -p /var
-		rm -f /var/crash
-		mv /root/var/crash /var/crash
-	fi
+	remove_mfs_link /root /var/cache opnsense-update
+	remove_mfs_link /root /var/cache pkg
+	remove_mfs_link /root /var/db pkg
+	remove_mfs_link /root /var crash
 fi
 
 # make some directories in /var