linux: Install the device tree in /boot when installing the kernel

The option BR2_LINUX_KERNEL_INSTALL_TARGET used to install the kernel in
/boot in the rootfs. The introduction of the device tree builds made it
possible to generate device tree blobs, and stored them in IMAGES_DIR
like the kernel, but didn't copy the dtb in /boot when _INSTALL_TARGET
was enabled.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>