Add two variables to adjust where to install kernel and modules.

Add two variables to adjust where to install kernel and modules.

DESTLABEL	common suffix added to kernel and modules directory.  for
		example, `make DESTLABEL=test installkernel' installs them
		as /boot/kernel.test and /boot/modules.test, respectively.

DESTKERNDIR	where to install kernel and modules, /boot by default.

Add /sys/conf/kern.path.mk to define default paths for kernel and modules.
Also move KMOD* definitions to /sys/conf/kmod.mk where it really belongs.

Submitted-by: Stefan Johannesdal <stefan.johannesdal@gmail.com>
with a slight modification by me.