This is a follow-up of f19cf27c2b. The
bootloader installation must be done on the final disk-image, hence using
"disk-image-installer" instead of "installer" callback.
* gnu/bootloader/u-boot.scm: Turn all installer callbacks into
disk-image-installer callbacks.
* gnu/build/bootloader.scm (write-file-on-device): Open the output file with
'no-truncate and 'no-create options.
* gnu/system/image.scm (with-imported-modules*): Add (gnu build bootloader)
module.
b97b423e3f