build: don't add -j for parallel builds with Make 4.2+

Make usually passes -j and jobserver arguments as part of MAKEFLAGS. LEDE
removes MAKEFLAGS to have better control of the build, and re-adds the
jobserver arguments with -j to the Make commandline where desired.

Make 4.2+ behave differently with these arguments passed on the commandline
than in MAKEFLAGS: -j will override the jobserver argument and the job
count will be unlimited.

Moving the flags to MAKEFLAGS will need many packages to be changed and
tested; therefore, we opt for a less invasive change for now and just
remove -j for Make 4.2+, as the jobserver argument alone is enough to
enable parallel builds for these Make versions.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>