Install backport during netinstall installation process

[Rory Campbell-Lange <rory@campbell-lange.net>]

I've been struggling to fully setup some machines with A2SDi-8C-HLN4F
motherboards and Intel x553 NICs (which require recent ixgbe support) as
Debian stable machines.

Thanks to considerable help from this list, I've been able to
successfully setup the machines over the net using buster/testing
netinstall images, which allow one to select stretch/stable at install
time, then reboot into rescue mode on the netinstaller and install
kernel-image-4.19.0-0.bpo from backports to provide a stable system and
kernel with ixgbe support.

However this doesn't work when running root off a RAID1 array with
dm_crypt on top. Rescue mode can't both reassemble the array and decrypt
the volume. While I can do that from the rescue shell I don't know how
to pivot to make the decrypted volume root so I can install the backport
kernel. **

Consequently I'd be grateful to know if:

* it is possible to somehow install the backport kernel and dependencies
  during the netinstall process 
  (apt-get install doesn't seem available on the virtual terminals)

* alternatively, what mount point and commands should I use to make a
  root file system (which I've re-raided and decrypted) available to the
  buster installer in rescue mode

Thanks very much
Rory


** there is another issue, which is the stretch grub doesn't seem to be
able to decrypt a dm_crypt volume when made from buster. I've reported
that to debian-boot.