Prevent packages uninstall & reinstall after restoring a config?

[carlm]

pfSense nanoBSD 2.2.5

Is there a way to prevent pfSense from uninstalling and reinstalling all packages when it reboots after a config restore?
I'm not talking about a pfSense version upgrade or a fresh install of a new pfSense version – just a simple config restore.
I have good reasons for wanting to skip package reinstallation.

One thought I had was to restore a config.xml with 'installedpackages' removed. On reboot, boot single user and replace with the normal config.xml.

Would that work? Is there a simpler way? What tells pfSense, when it reboots, to uninstall and reinstall all packages? Is there a flag file I could delete if I booted single user on reboot?

Thanks!

EDIT:

Messing with config.xml as proposed above is obsolete because Jim's suggestion in the following post is simpler and it works (tested).
However, for the record, I didn't describe the process correctly.
First reboot: Boot normally. pfSense finds no installed packages (I think, not tested), so no uninstall/reinstall.
Second reboot: Boot single-user and replace the no-package config.xml with the normal config.xml with 'installedpackages'.
THIS IS NOT TESTED. USE PROCEDURE IN FOLLOWING POST.

[jimp]

From the shell:

# /etc/rc.conf_mount_rw
# rm /conf/needs_package_sync

[carlm]

@jimp:

From the shell:

# /etc/rc.conf_mount_rw
# rm /conf/needs_package_sync

Thanks!
Booting single-user and deleting /conf/needs_package_sync did the trick.
I can now restore a config without having 30+ minutes of downtime.

I did it slightly differently, though.
I didn't know if using PHP would have worked in single-user mode.
Instead, at the shell prompt:

/sbin/mount -w -o sync,noatime /dev/ufs/cf /cf

rm /conf/needs_package_sync

/sbin/umount /cf

exit

After exiting the single-user shell the system proceeds to boot multi-user.

WARNING: Do not use this trick if the pfSense version to which you are restoring config.xml is not the same as the pfSense version from which the config.xml backup was made.