Summary

Currently in the esp32 port the size of the SPI flash must be configured at build time, eg 4MiB, 8MiB, etc. Also, the esp32 partition table must be configured at build time, which depends on the size of the SPI flash. A bigger flash means more can be allocated to the user filesystem.

This PR makes it so the SPI flash size is automatically determined at runtime, and the filesystem size is automatically set to take up as much room as possible.

This works by:

1. Setting the SPI flash size to be 4MiB in the build.
2. Removing thevfs partition from the esp32 partition table (only nvs, phy_init and firmware remains in the partition table).
3. At boot, query the physical size of the SPI flash and use that as the actual size in the code.
4. Automatically create aautofs partition which takes up the flash from 2MiB up to the end of flash.

This allows simplifying a lot of board configurations, and removing some board variants that just change the flash size.

It's also fully backwards compatible, in the following sense:

1. Existing boards with MicroPython firmware will continue to work with the same filesystem, ie the filesystem won't be erased when the firmware is updated.
2. If a user has a custom esp32 partition table and installs MicroPython as a bare app into the app partition, the new MicroPython firmware will honor the esp32 partition table and use eithervfs orffat partitions as the filesystem.

Eventually this mechanism will be extended:

• to other ports, probably rp2 next
• to allow users to customise the filesystem layout, and enable ROMFS partition(s) without rebuilding firmware

Testing

Tested with ESP32_GENERIC on a board with 4MiB of flash, and ESP32_GENERIC_S2 on a board with 16MiB flash. The filesystem size was correctly detected.

Trade-offs and Alternatives

• Boards configured for OTA have not been changed, they still define the filesystem partition explicitly.
• It may be that the physical flash size cannot be detected correctly, but I think the IDF has covered most cases by now.
• Custom board definitions can override the auto detect feature by defining their ownMICROPY_BOARD_STARTUP macro.