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for these extra kernels so that, e.g. /boot/kernel.SHIVA, will get properly kldxref'd upon update and avoid outdated linker.hints.