Security v1-v2 transition has been completed for a while and path
separation is no longer needed. However, this commit preserves
`ndn::security::v2` namespace, but makes it inline.
Some commonly used v2 headers are preserved, but are deprecated and
should be avoided in the dependent software.
Change-Id: I26056cb3d34bcdc0a16e06dea8c1a78c03da2d40
Header files are now always included with their path from the
project's root directory rather than the path relative to
the includer.
Change-Id: If39b8510eef8da00baf5fe88337c45dbdf8c766e
Refs: #3084