Merge r293282 from head:
Provide knob NO_INSTALLEXTRAKERNELS. If defined, extra kernels in KERNCONF
won't be installed, only the first one would.

Turn NO_INSTALLEXTRAKERNELS=yes on stable/10 to preserve original
behaviour of not installing additional kernels built.

Reported & tested by: Trond Endrestøl <Trond.Endrestol fagskolen.gjovik.no>
Reported & tested by: dhw