Correct the usage of $(DESTDIR) and $(PREFIX)

Correct the usage of $(DESTDIR) and $(PREFIX)

The GNU coding standards expect $(DESTDIR) to be the root of everything
installed, and for $(PREFIX) to then be added to the path.  This is not how
XTF previously behaved.

XTF is not a typical package, and doesn't meet the usual semantics; it expects
to arrange all files under a single directory.  Drop the use of $(PREFIX)
entirely (to avoid the expectation that it behaves as $(prefix) usually
behaves) and introduce $(xtfdir) instead.

$(DESTDIR) now works as intended for staged installes, and $(xtfdir) is the
single selected directy containing all installed content.

The intended way to install XTF now:

  $ make install DESTDIR=/path/to/staging/area xtfdir=/local/scratch/xtf

Reported-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>