This is now possible on the new firmware versions (v0.9.x and later).
Access to the flash (NOR and NAND) from NVMe/USB/sdcard was disabled in the previous versions for multiple reasons:
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Older versions of some distributions contained a flash driver which was not able to access the NAND chip, and could corrupt it if used incorrectly
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At the time, it was anticipated that a solution for EFI variables via U-Boot would come out. The community has since decided on another method which does not touch the flash from the OS.
If you want, you can re-enable this in the earlier U-Boot by deleting the following portion of the setup_distroboot_efi variable:
fdt set "/soc/spi@20c0000" "status" "disabled"
(be careful attempting this remotely, the boot process will get stuck if not edited correctly)
I’m not aware of any method to re-enable it from a running operating system.