OpenROM: fix build and disable "CRT emulation" on startup for integrated fallback rom

[dansanderson]

When you switch on the MEGA65 with no SD card present in either slot, the v0.96 core uses default configuration and loads the embedded Open ROM. The default configuration has CRT emulation enabled by default, which is a little bit nice for the 40x25 on-boarding UI on a flat panel display, but it makes Open ROM's 80x50 display mostly illegible.

As polish, we should disable CRT emulation when booting Open ROM as a fallback.

1. The "No SD card" case starts Open ROM, I presume because it doesn't make sense to start on-boarding when config cannot be saved. Perhaps this is the only case that needs this change.
2. The "card present, no config" case starts on-boarding. If we still like CRT emulation on by default for this case (it's nice looking in common cases, makes a good first impression, can be disabled in on-boarding), then no change is needed for this path.
3. The "card has config but no ROM" case starts Open ROM (I think?). Naturally, we should honor the config setting in this case.

[lydon42]

Best solution without tinkering with the startup process at all is:
Make the integrated openrom clear the CRT emu flag. As OpenROM does not start for internal tools (MEGAFLASH), they still retain what was set in config.

In addition: fix or at least document build of open-roms, as currently we don't build the integrated openrom, we just use a prebuild version that was checked into the repo 2021.
I am fine with building mega65.rom as part of the build process, but then it should be made sure that the date of the pinned submodule commit is used to generate the version that is displayed.