dtbsinstall: don't move target directory out of the way

No other kernel installation target moves the target directory out of
the way, even deleting an old version of it.  These are destructive
operations, ones which the kernel build system should not be making.

This behaviour prevents being able to do:

	make install INSTALL_PATH=/some/path/boot
	make dtbs_install INSTALL_DTBS_PATH=/some/path/boot

As it causes the boot directory containing the kernel installed in
step 1 to be moved to /some/path/boot.old.  Things get even more fun
if you do:

	make install dtbs_install INSTALL_PATH=/some/path/boot INSTALL_DTBS_PATH=/some/path/boot

The kernel gets installed into /some/path/boot, then the directory gets
renamed to /some/path/boot.old, and a new directory created to hold the
dtbs.  Even more fun if you supply -j2 when we end up with races in
make.

Remove this behaviour.

If this behaviour is required at installation time, this should be
done by the installation external to the kernel makefiles, just lik...