Add another pi-config option

[sergey-borovkov]

This commit allows to use boot from usb functionality. It was implemented in the core at the end of previous year if I remember correctly but to use it this option is required according to this - https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md

[ogra1]

given that this option permanently changes the ROM code i'm not sure we should expose it in a generic config option you can run on any Pi.
i'd recommend to rather ship it in the gadget config.txt for this specific device and make the config-option carry over that setting when re-generating config.txt if it exists.

[chipaca]

Just a note to confirm that @sergey-borovkov has signed the CLA.

[zyga]

@ogra1 can you please explain how it is changing the rom code permanently?

[ogra1]

@zyga it toggles a register permanently ... some users might not want that so i'd prefer to not have it available as a generic option but rather somehow specifically set from the gadget itself

$ vcgencmd otp_dump | grep 17:
17:3020000a

is what is described in the documentation link above to find if it was toggled, to my knowledge there is no way to reset this back to factory defaults once you toggled it.

[zyga]

Based on what @ogra1 said I think it should be more carefully exposed. Perhaps we need a privileged interface or an interactive flow that allows this.

What is the use case? Anyone can toggle this manually (since it changes the board the OS is not important) and I cannot imagine this changing in the field.

[zyga]

-1, see my previous comment.