perf tools: Fix 'make help' message error

perf tools: Fix 'make help' message error

Currently 'make help' message has such hint:

   use "make prefix=<path> <install target>" to install to a particular
       path like make prefix=/usr/local install install-doc

But this is misleading, when I specify "prefix=/usr/local", it has got no
respect at all.

This is because that, "DESTDIR" is considered first. In this case, "DESTDIR"
has an empty value, so "prefix" is honored. However, "prefix" is unconditionally
assigned to $HOME, regardless of what it is set to from command line. So our
"prefix" setting got no respect and the actual destination falls back to $HOME.

This patch fixes this issue and corrects the help message.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1401727474-19370-1-git-send-email-nasa4836@gmail.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>