LU-18251 build: annotate lustre.spec

LU-18251 build: annotate lustre.spec

Consolidate duplicate branches and use some comments
to clarify nested if/endif pairs.

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ic30b8c0c617c65ffc9096f3cb1e4338c104cf0b2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56448
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Caleb Carlson <caleb.carlson@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>