make 3.82 splits the work differently. When using buildroot with
the make > 3.81 buildroot uses BR2_JLEVEL= (preferably 0 for automatic number
of jobs setting).
For make =< 3.81 BR2_JLEVEL= is ignored and build is done using only one job.
Passing -j allows to build with many jobs, but many warning messages are
printed:
make[2]: warning: -jN forced in submake: disabling jobserver mode.

Use BR_JLEVEL=0, this allows buildroot to pick automatically number of jobs.
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>