automatically set PARALLEL_JOBS when BR2_JLEVEL is 0

When BR2_JLEVEL is 0, set PARALLEL_JOBS to double the number of CPUs
detected.  This allows one to more or less fully utilize the host
system without manually tuning the configuration.

Also make 0 the default value for BR2_JLEVEL.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>