Add DTS as regular file, not as patch

[imphil]

Currently the only way device tree files can be added is through patches. A small change in buildroot would allow them to be added as regular file, which is much more convenient when editing (or generating) dts files.

Wait for feedback on http://lists.busybox.net/pipermail/buildroot/2018-November/235945.html.

If this patch goes in the configuration could look like this:

BR2_LINUX_KERNEL_CUSTOM_DTS_PATH="$(BR2_EXTERNAL_OPTIMSOC_PATH)/board/compute_tile_singlecore/compute_tile_singlecore.dts"
BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT=y

[imphil]

This patch is now merged and should be in the upcoming 2019.08 release.