Booting - User device selection in U-Boot?

[Arwen]

One of the issues that comes up is that we can't easily select what device to boot during startup.
U-Boot does not have that option for us, yet. During testing, I've had to remove and insert the
MicroSD card far too many times. It would be nice if I could leave it installed and force booting
something else.


So, how about something like this?

U-Boot selectable device target:
        - Pause & blink LED, (which one?), for a short time, (5 seconds?)
        - During this time, accept keyboard input
        - Select boot device of:
                - eMMC, (number 1?)
                - MicroSD card, (number 2?)
        - Timeout default using current scheme, MicroSD if installed, eMMC otherwise.

The exact keys for which devices are not really important. And if NVMe or USB booting works fine
then those could be added as well. Plus, allow changing the default boot order at install time, (so
people with NVMes could be a priority over eMMC).

Plus, perhaps having the ESC and Enter keys immediately jump to the default action. So a way to
skip the pause.

As time goes one, perhaps we can add a primitive text menu on the screen.


At present, this idea does not care where U-Boot is installed, (SPI, eMMC, MicroSD card). For
testing, it probably easiest to have it on a eMMC or MicroSD card. This idea is to address which
device we use for "/boot".

So, what does anyone else think?

Perhaps someone has a better idea?

Does using Grub as a target allow boot chaining?
(Meaning a grub menu entry can say go to another device.)