noenvdoes not save environment anywhere
spisaves the environment to the SPI device
mmcbootdoes not save environment anywhere
Only the built-in environment is used, read-only. In other words,
does nothing useful.
This variant is to be used with shared firmware storage retrofitted with a Tow-Boot installation. In other words, in a system that does not provide a protective partition for the firmware.
This variant is also used to build the installer images, as it ensures no stray environment will change the behaviour of the installer when booted form the embedded Tow-Boot.
NOTE: This variant is currently used on all shared firwmare storage installations. In other words, using the protective partition does not currently grant the ability to save the environment.
This variant is to be used on systems with SPI Flash as dedicated firmware storage.
This variant is to be used on systems with eMMC Hardware boot partitions as dedicated firmware storage.
It is expected that two other types of variants will come in the future. They will provide environment facilities for the shared firmware storage installations with protective partitions.
One will save the environment at the end of the protective partition.
The other, spcecialized for the Raspberry Pi family of hardware, will save the environment as a file on the Tow-Boot firmware partition.