Implement SRAM save/load support

[kbeckmann]

After #12 is done, look into how to backup the SRAM after the system has been running. Some solutions seem to trigger SRAM saving by holding the reset button on the N64 (N64_COLD_RESET is held low during this time I think).

Write code that saves the SRAM to external flash.

Write code that can load an initial SRAM image before booting the software.

For now, let's focus on using the external flash, and SDCard/WiFi later on. Let's worry about the trigger later (N64_CR or something else).

[kbeckmann]

Trigger SRAM commit when NMI goes low (this happens when reset is pressed). Keep NMI asserted until flash write is done.

[kbeckmann]

This is implemented