Implement simple uart-based updater for the bootrom

Created by: grg-haas

In Keystone (https://github.com/keystone-enclave/keystone), we'd like to be able to run automated CI tests for all of our platforms to make sure that code changes don't break things, as much as possible. One difficulty is that many boards don't support updating firmware remotely. For CVA6 specifically, the firmware update process involves removing the microSD card, flashing a new image, and inserting it again. This does not work well for our remote tests.

This PR implements a simple UART-based updater in the bootrom. By hitting a key during boot, a user can then transfer an image of a specific size directly to memory and boot it. We plan to transfer a uboot image with TFTP capabilities, allowing us to then quickly flash the much bigger OS image as well.