bootloader: Fix u-boot installation.

bootloader: Fix u-boot installation.

This is a follow-up of f19cf27c2b9ff92e2c0fd931ef7fde39c376adaa. 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.