[U-Boot] How to update Espressobin (U-Boot) firmware?

[Andre Przywara]

Hi,

I wanted to exploit U-Boot's EFI support on the Espressobin board, to
use off-the-shelf arm64 distribution installer images. This should work
with current U-Boot, which has support for the Espressobin board (and
for UEFI).

The firmware my board ships with seems to be built from older and
non-upstream sources - and, in contrast to the Macchiatobin board - I
cannot find either proper sources for all components (mv-ddr, ATF) nor
*any* kind of instructions how to (re-)build and update the firmware.

So I was wondering if there are any instructions or documentation I
missed on how to update at least U-Boot?

I see some hints that people are apparently chain-loading mainline
U-Boot, is this the only way?
I dumped the SPI flash and could identify the ATF FIP image on it, but I
am a bit wary of bricking the board by just squashing a freshly built
u-boot.bin into that - which I guess wouldn't work that easily anyway.

And to allow experimentation: Are the boot selection jumpers actually
working? Setting them to SD card (111) doesn't seem to make a difference
(it still booted off the SPI flash, apparently). Or does it fall back to
SPI flash in case the SD image is not valid (I put the dumped image at
sector 1)?
I see that selecting "UART" (110) gives me that "E" prompt, apparently
waiting for some data to be fed, but again can't tell what to do from
here? I take it that mkimage/kwboot won't work as is, as it needs ATF? I
found some mentioning of a Windows binary, but hope that's not the only
way to use this mode?

I'd be very grateful for any insight into this!

Cheers,
Andre.