openhcl_boot: add ability to store logs and report them via devicetree to usermode

[chris-oo]

It's kind of annoying to have to enable COM3 sometimes and setup your vm to enable logging and such. It would be nice to add an environment variable to allow saving logs to the device tree passed to usermode, so you can inspect them later.

This would also allow us in the future to potentially save bootshim logging as well, since we don't normally enable com3.