support CIRCUITPY_SDCARD_USB in settings.toml

[dhalbert]

• Implements a new setting in settings.toml: CIRCUITPY_SDCARD_USB.
  • if true (the default), presents a mounted SD card (auto or manually mounted) via USB MSC
  • if false, the SD card is not presented over USB.
• Revises docs/environment.rst to bring it up to date and clarify how it works. Documents CIRCUITPY_SDCARD_USB as well.

Tested on a Fruit Jam, Metro ESP32-S3, and PyPortal.

@tannewt FYI:

I deleted the lines belowin tud_msc_test_unit_ready_cb, because they did not seem to be necessary. There are other places where automount_sd_card() is called which cause an SD card to be mounted, and I'm not sure why it's called here too.

circuitpython/supervisor/shared/usb/usb_msc_flash.c

Lines 375 to 380 in e512a25

	#ifdef SDCARD_LUN
	if (lun == SDCARD_LUN) {
	automount_sd_card();
	}
	#endif

I tested this deletion on a Fruit Jam: the SD card is still mounted (when CIRCUITPY_SDCARD_USB is not false) when code.py runs, and also when there is not even a code.py to run. Let me know if you remember what use case this was covering that I missed.

Tagging @bablokb and @mikeysklar for interest re #10983, and also for testing if you would like to.

[tannewt]

One typo. Good otherwise.