Ubuntu: Use grub-efi-amd64's multiple disk code

Ubuntu 20.04's GRUB supports multiple EFI disks.  There is a small
caveat in that it doesn't prompt in the chroot, but it works fine after
the reboot.  Using the stock support means that the ESPs will be kept in
sync automatically.

Signed-off-by: Richard Laager <rlaager@wiktel.com>
Refs issue #55

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -767,8 +767,8 @@ Step 4: System Configuration
          /boot/efi vfat defaults 0 0 >> /etc/fstab
      mount /boot/efi
 
-   For a mirror or raidz topology, repeat these steps for the additional
-   disks, using ``/boot/efi2``, ``/boot/efi3``, etc.
+   For a mirror or raidz topology, repeat the `mkdosfs` for the additional
+   disks, but do not repeat the other commands.
 
    **Note:** The ``-s 1`` for ``mkdosfs`` is only necessary for drives which
    present 4 KiB logical sectors (“4Kn” drives) to meet the minimum cluster
@@ -815,7 +815,9 @@ Step 4: System Configuration
            shim-signed zfs-initramfs zsys
 
      **Note:** For a mirror or raidz topology, this step only installs GRUB
-     on the first disk. The other disk(s) will be handled later.
+     on the first disk. The other disk(s) will be handled later.  For some
+     reason, grub-efi-amd64 does not prompt for ``install_devices`` here, but
+     does after a reboot.
 
 #. Optional: Remove os-prober::
 
@@ -961,14 +963,6 @@ Step 5: GRUB Installation
        grub-install --target=x86_64-efi --efi-directory=/boot/efi \
            --bootloader-id=ubuntu --recheck --no-floppy
 
-     For a mirror or raidz topology, run this for the additional disk(s),
-     incrementing the “2” to “3” and so on for both ``/boot/efi2`` and
-     ``ubuntu-2``::
-
-       cp -a /boot/efi/EFI /boot/efi2
-       grub-install --target=x86_64-efi --efi-directory=/boot/efi2 \
-           --bootloader-id=ubuntu-2 --recheck --no-floppy
-
 #. Disable grub-initrd-fallback.service
 
    For a mirror or raidz topology:
@@ -1046,6 +1040,15 @@ Step 6: First Boot
 
    Wait for the newly installed system to boot normally. Login as root.
 
+#. Install GRUB to additional disks:
+
+   For a UEFI mirror or raidz topology only::
+
+     dpkg-reconfigure grub-efi-amd64
+
+     Select (using the space bar) all of the ESP partitions (partition 1 on
+     each of the pool disks).
+
 #. Create a user account:
 
    Replace ``username`` with your desired username::