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.

at the moment, all updates are executed in form of shell scripts. these are error-prone and hard to reason about in a comprehesive way once the codebase gets sufficiently large. the short term goal is to migrate shell scripts to something more managaeble like saltstack but with less resource requirements, suitable for embedded devices without python dependencies.

typical update examples are: upgrade bitcoind, lnd and other services, 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.