pkgbase: Fix post-install script for kernel packages

kernel.ucl uses a hardcoded boot/kernel for kldxref, which is the incorrect
directory when we're installing extra kernels that aren't the "default"
kernel (placed at boot/kernel).

Fix this by instead using a new %KERNELDIR% that we now replace in
Makefile.inc1 with "kernel" for the default kernel and "kernel.${_kernel}"
for these extra kernels so that, e.g. /boot/kernel.SHIVA, will get properly
kldxref'd upon update and avoid outdated linker.hints.

Reviewed by: gjb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14689