system updates

the plan is for this repo to contain all system updates, incremental in a form of text/source code. a node periodically runs the update.sh script which pulls the repo to receive updates executes apply.sh. the latter then makes changes and updates the operating system.

typical examples are: upgrade bitcoind, lnd and other services, update system packages, improve configuration of components such as firewall. the run sequence on the node is approximately as follows:

1. fetch updates with a git fetch.
2. provide a git diff on the screen and confirm with the user.
3. pull in the changes with a git pull --verify-signatures.
4. run apply.sh.

at the moment, an on-screen diff and confirmation aren't implemented yet. nd and ngui is where it'll happen, in the ndg repo.

when configuring a new node, clone this repo and set up a cron job to execute the update.sh script once a day. The script requires REPODIR and LOGFILE env variables set.

TODO: add a list of supported platforms; the "native" is void linux.