Makefile.inc1: Make NO_INSTALLKERNEL less of a hack

Rather than adding a dummy entry which requires everyone to know to skip
it, introduce a new INSTALLEXTRAKERNELS which contains the set of
kernels to install as kernel.KERNCONF.

Reviewed by: ivy
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54291

(cherry picked from commit 2c0e9a76c1cee8766245a0b5bdad73e995b790c6)