Make bootgen-xlnx a transitional package without real contents

Need to update xlnx-frimware before removing it from lunar.
I'll update xlnx-frimware to replace bootgen-xlnx with xilinx-bootgen.

---
Package: xlnx-firmware
Depends: ${misc:Depends}, u-boot-zynqmp, bootgen-xlnx

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Typically, the way I've seen this done in the past is actually a bit simpler than this:

Rather than changing bootgen-xlnx to have a different source format, and removing all the files, and changing the d/rules and so on ... leave this package as it is, and define a new (empty, obviously) bootgen-xlnx transitional binary package in the intended target. This does assume that the intended target has a version greater than the "old" package but that is the case here.

In other words, we do nothing to the bootgen-xlnx *source* package, leaving it alone at version 2022.1-0ubuntu1. In the xilinx-bootgen source package, we add the (empty) transitional entry for bootgen-xlnx to d/control so xilinx-bootgen source version 2022.2-2 will generate an (empty, transitional) bootgen-xlnx 2022.2-2 which will supercede the current bootgen-xlnx 2022.1-0ubuntu1.

This keeps the changes to bootgen-xlnx minimal (in fact, empty), and results in minimal changes to xilinx-bootgen (one extra, empty, binary package). It will be a delta to Debian in xilinx-bootgen but one that can be dropped at the next cycle.

I'm going to add xilinx-bootgen as affected here as the transitional package will actually be defined there.

This bug was fixed in the package xilinx-bootgen - 2022.2-2ubuntu1

---------------
xilinx-bootgen (2022.2-2ubuntu1) lunar; urgency=medium

  * d/rules, d/control: Place Breaks, Replaces in d/control and add
    transitional definition for xlnx-bootgen. This delta can be removed after
    lunar's release (LP: #1998728, LP: #2000774)

 -- Dave Jones <email address hidden> Thu, 12 Jan 2023 17:45:38 +0000