bootloader: Add extlinux support.

bootloader: Add extlinux support.

* gnu/bootloader.scm: New file.
* gnu/bootloader/extlinux.scm: New file.
* gnu/bootloader/grub.scm: New file.
* gnu/local.mk: Build new files.
* gnu/system.scm: Adapt to new bootloader api.
* gnu/scripts/system.scm: Adapt to new bootloader api.
* gnu.scm: Remove (gnu system grub) and replace by (gnu bootloader) and (gnu
bootloader grub) modules.
* gnu/system/grub.scm: Moved content to gnu/bootloader/grub.scm.
* gnu/system/vm: Replace (gnu system grub) module by (gnu bootloader).
* gnu/tests.scm: Ditto.
* gnu/tests/nfs.scm: Ditto.