Ubuntu/Debian: Fix bpool importing

We need the bpool to end up in zpool.cache.  For some reason, this is
not happening normally on Ubuntu 20.04.  I added the same steps to
Debian too, for good measure.

This was discussed in #120.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Debian/Debian Buster Root on ZFS.rst

@@ -226,6 +226,7 @@ Step 2: Disk Formatting
 #. Create the boot pool::
 
      zpool create \
+         -o cachefile=/etc/zfs/zpool.cache \
          -o ashift=12 -d \
          -o feature@async_destroy=enabled \
          -o feature@bookmarks=enabled \
@@ -528,6 +529,11 @@ Step 3: System Installation
    An alternative to using ``debootstrap`` is to copy the entirety of a
    working system into the new ZFS root.
 
+#. Copy in zpool.cache::
+
+     mkdir /mnt/etc/zfs
+     cp /etc/zfs/zpool.cache /mnt/etc/zfs/
+
 Step 4: System Configuration
 ----------------------------
 

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

@@ -383,6 +383,7 @@ Step 2: Disk Formatting
 #. Create the boot pool::
 
      zpool create \
+         -o cachefile=/etc/zfs/zpool.cache \
          -o ashift=12 -o autotrim=on -d \
          -o feature@async_destroy=enabled \
          -o feature@bookmarks=enabled \
@@ -655,6 +656,11 @@ Step 3: System Installation
    An alternative to using ``debootstrap`` is to copy the entirety of a
    working system into the new ZFS root.
 
+#. Copy in zpool.cache::
+
+     mkdir /mnt/etc/zfs
+     cp /etc/zfs/zpool.cache /mnt/etc/zfs/
+
 Step 4: System Configuration
 ----------------------------