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.
* gnu/build/bootloader.scm (install-efi): New procedure copied from (gnu build vm).
(install-efi-loader): New exported procedure, wrapping install-efi.
* gnu/build/vm.scm (initialize-hard-disk): Adapt to use install-efi-loader.
* gnu/bootloader/extlinux.scm (install-extlinux): Factorize bootloader
writing in a new procedure write-file-on-device defined in (gnu build
bootloader).
* gnu/build/bootloader.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add new file.
* gnu/system/vm.scm (qemu-img): Adapt to import and use (gnu build bootloader)
module during derivation building.
* gnu/scripts/system.scm (bootloader-installer-derivation): Ditto.
