Revision M+ support

[calc84maniac]

Makes the following changes to support new hardware on rev M+ and lay groundwork for future Python support and LCD improvements:

• Added serial Flash chip support (commands used by the OS are supported)
• Added Flash cache support with appropriate wait states
• Fixed some aspects of parallel Flash support, like resetting it properly to avoid unrecoverable state when misconfigured
• Improved SPI emulation and added UART emulation, both with accurate transfer timing, loopback support, and FIFO/interrupt behavior
• Added SPI chip select support (allows deselecting the LCD on Python editions)
• Added basic support for interlaced scan and MCU/VSYNC display modes (inaccurate timing but should display all pixels now)
• Revision selection in settings rather than previously scattered options like IM 2 enable
• Automatic revision selection based on the detected boot code version, also restricts usable revisions unless a debug option is set
• Automatic Python edition detection based on the certificate, can be overridden with the same debug option
• Flash statistics (average cycles per access and number of cache misses) added to debug cycle counting view

[calc84maniac]

I've discovered an unusual bug in rev M emulation, which I think is a blocker on this PR.

Basically, the issue is that if you turn off the calculator while emulating rev M, linking stops working. This issue persists through resets and state saves/loads, and even when resetting into another revision. The only way to restore linking is to reload the ROM.

I did a git bisect, and found that this issue exists even back in @jacobly0's first commit, which leads me to believe it's something related to different code paths (in the boot code?) executed on rev M. But I don't understand why the problem persists through resets and other revisions. Perhaps there's some USB register that's never getting reset? I would appreciate if an expert on linking could look into this issue.

[calc84maniac]

Finally fixed the outstanding USB issue, last call for reviews / sanity testing

[adriweb]

@calc84maniac said it's ready and people have tested it without issues so far, let's merge :)