LU-12634 build: Recognize ELRepo -ml mainline kernel

LU-12634 build: Recognize ELRepo -ml mainline kernel

Add support for identifying ELRepo kernel-ml style
builds on CentOS 7 and 8 based distributions

Test-Parameters: trivial
Cray-bug-id: LUS-7689
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: If4ae7441d4d023d31b1fb42f3fe90ff9c747c0f8
Reviewed-on: https://review.whamcloud.com/35742
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>