* 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.