LU-17881 build: fix _flavor definition

LU-17881 build: fix _flavor definition

Allow user to override _flavor definition.
Move ordering of _kver definition to be fully
defined before using it to define _flavor.
This prevents _flavor from getting defined with
the kernel patch version field.

Signed-off-by: Caleb Carlson <caleb.carlson@hpe.com>
Test-Parameters: trivial
HPE-bug-id: LUS-12267
Change-Id: Ibd4db360d8c16f487453593cb0a9fd2a6a5a8c62
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55225
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>