package/Makefile.in: don't force jobs when BR2_JLEVEL is empty

When the "BR2_JLEVEL" variable is empty use "make" without
the "-j" option, because to be able to use top-level parallel
make we must not force the number of jobs in sub-make.

Example:
	make BR2_JLEVEL= -j8

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>