backports: move version file generation to run earlier

backports: move version file generation to run earlier

The version file contains:

  * BACKPORTS_VERSION
  * BACKPORTED_KERNEL_VERSION
  * BACKPORTED_KERNEL_NAME

This file is used by the Makefile to export variable
definitions used to identify the backport. For kernel
integration we'll be using a generated Kconfig file
instead and since these are needed when processing
kconfig logic this will need to be defined earlier.

Just move the generation of the version file for packaging
up early to match where we will generate the Kconfig with
the backports versioning identification.

This change has no real functional change other than the
move of some code running earlier.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>