linux: Build $KERNEL_UBOOT_EXTRA_TARGET before kernel image

When there is a need to build the kernel with appended device tree for compatibility with older bootloaders,
$KERNEL_UBOOT_EXTRA_TARGET, which usually is a list of device tree source files, needs to be built
before the kernel, making device tree binaries available for appending to the kernel image.
Merge pull request #3701 from codesnake/kernel_with_appended_dtb

linux: Build $KERNEL_UBOOT_EXTRA_TARGET before kernel image