Just looking for config-bisect.pl in the source tree can be risky,
especially, if the source tree being tested doesn't have config-bisect.pl in
place. Instead, allow the user to set where to find config-bisect.pl with a
new option CONFIG_BISECT_EXEC.

If this option is not set, by default, ktest.pl will look for
config-bisect.pl in the following locations:

 `pwd`/config-bisect.pl # where ktest.pl was called from
 `dirname /path/to/ktest.pl`/config-bisect.pl # where ktest.pl exists
 ${BUILD_DIR}/tools/testing/ktest/config-bisect.pl
   # where config-bisect.pl exists in the source tree.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>