Merge r299077, which provides ability to override NO_INSTALLEXTRAKERNELS.

Override NO_INSTALLEXTRAKERNELS for the release build. Historically, in
the stable/10 branch the regular 'installkernel' target DID NOT install
extra kernels, while the release build DID. This change finally restores
POLA, I hope.

Reported by: Fedor <f att.org.ru>
Reviewed by: gjb