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>
cfa-10036: Update barebox environment

We are no longer using the boot partition but instead retrieve the
kernel image and the device tree from the rootfs. Update the custom
environment and configuration to reflect this behaviour.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
cfa-10036: Update the post-build and flash scripts for the new layout

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>