perf tools: do not look at ./config for configuration

In addition to /etc/perfconfig and $HOME/.perfconfig, perf looks for
configuration in the file ./config, imitating git which looks at
$GIT_DIR/config.  If ./config is not a perf configuration file, it
fails, or worse, treats it as a configuration file and changes behavior
in some unexpected way.

"config" is not an unusual name for a file to be lying around and perf
does not have a private directory dedicated for its own use, so let's
just stop looking for configuration in the cwd.  Callers needing
context-sensitive configuration can use the PERF_CONFIG environment
variable.

Requested-by: Christian Ohm <chr.ohm@gmx.net>
Cc: 632923@bugs.debian.org
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Christian Ohm <chr.ohm@gmx.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110805165838.GA7237@elie.gateway.2wire.net
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>