introduce PARALLEL_JOBS build system variable

BR2_JLEVEL currently is expanded directly in $(MAKE), and used in
invocations of other build software (e.g. ct-ng).  However, we are
going to allow "0" to be a meaningful value for BR2_JLEVEL, which
won't work for these uses.  Given that it is not permissible to modify
BR2_-prefixed variables in Makefiles, we need an intermediate
variable.

Define PARALLEL_JOBS to $(BR2_JLEVEL), and use the former in MAKE's
definition.  Uses of BR2_JLEVEL throughout the rest of the tree to be
adjusted similarly in follow-up patches.

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