Hello, as it was after discussion with Ludovic that he posted this bug 
report, let me express my opinion as a simple user.

it's just a matter of consistency: very basically, the examples mention 
grub-bootloader. With the default image type (efi-raw), it's been 
working perfectly for years (maybe by chance, but in any case it seemed 
to be a compatible combination). The recent change means that it no 
longer works :-).  What's more, the error message isn't very explicit, 
and doesn't point to a configuration error but to what may appear to be 
a recently-introduced bug. Switching to grub-efi-bootloader allows you 
to build the image (and start it).

I don't know if it's simply possible to have a consistency check between 
the image type and the bootloader used: "Bootloader probably not 
compatible with image type, please use grub-efi-bootloader".

Maybe just change the doc to include grub-efi-bootloader in the 
examples. Or indeed have a format that remains compatible with MBR 
partitioning and the appropriate grub.

-- 

Yann