make mounting /tmp as tmpfs configurable and support device defaults for it

I was thinking about this again recently, and perhaps there's a way to support this using a deviceinfo variable to configure it. This would allow device maintainers to set it by default for devices with "enough" RAM (where having /tmp as tmpfs wouldn't make firefox, etc unusable due to low available RAM), and it would be a switch that users could use to toggle it from the default for their device, by setting the var in /etc/deviceinfo.

The var would be used by a new service that runs on boot, that would enable it. The var would be a bool, or take a size param (either as % of available memory, or set GB or ??). I'm not sure if some service like this already exists, like with zram-init, but what I had in mind would operate similarly to zram-init.

related: #1342 (comment 1200897723)

(cc @stacyharper)