bootstrap: attempt to reinstall plugins registered in config.xml

[fichtner]

see https://forum.opnsense.org/index.php?topic=29323.0

[fichtner]

A brief look shows that this has some constraints... reading the config.xml, (re-)registering plugins and all of this using PHP code. /usr/local/opnsense/scripts/firmware/sync.subr.sh is available but we need to check if this works reliably when bootstrapping a new system and all the libraries and PHP modules are loaded correctly.