Add 2.17 release info

Merge pull request #7355 from xcat2/master

Merge from master to 2.16 branch for 2.16.5 release. (1)

Merge pull request #7199 from xcat2/master

Merge from master to 2.16 branch for 2.16.4 release. (3)

Tag 21C release

Merge pull request #7066 from xcat2/master

Merge from master to 2.16 branch for 2.16.3 release. (1)

Fix xnba problem with legacy boot

Merge pull request #6987 from xcat2/master

Merge from master to 2.16 branch for 2.16.2 release. (3)

Lenovo amended xCAT 2.16.1

Merge pull request #6874 from xcat2/master

Merge from master to 2.16 branch for 2.16.1 release. (2)

Fix pxelinux.0 dependency

We don't actually use pxelinux.0 for the most part,
so only copy if it exists.