Implement system-update feature

The `system-update` and `install-update` scripts are the same as from
Aimee OS v1, with references to Gentoo replaced, of course.  We need
some additional kernel features in order to mount the firmware partition
and update the GRUB environment file, and of course the `grub-editenv`
tool itself.  We also need `wget` for now, since that's how the tool
downloads the specified update file from the network.

Eventually, `system-update` will be replaced by a much more robust tool,
with package URL discovery, signature verification, etc.  The shell
script will do for now while development is still proceeding rapidly.

ci/Jenkinsfile

@@ -65,6 +65,7 @@ pipeline {
                             'firmware.img.zst',
                             'rootfs.squashfs',
                             'sdcard.img.zst',
+                            'update.tar.zst',
                         ].join(','))
                     }
                 }