Add NetBSD 5.1.4, 5.2.2 & 7.0.1 releases to the tree.
Ammend the position of NetBSD 6.0.2 release in the tree as it came
after OpenBSD[1] & DragonFlyBSD[2] release according to the release
information.
The entries for the 6.0.5 & 6.1.5 releases were incorrect (fetched from
NetBSD CVS copy) and confirmed with history page[3]

[1] http://www.openbsd.org/53.html
[2] https://www.dragonflybsd.org/releases/
[3] http://netbsd.org/releases/formal.html#history

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D8099