[Buildroot] [PATCH next] uboot: support external DTB in U-Boot images

[Matthew Weber <matthew.weber@rockwellcollins.com>]

Clemens,

On Mon, Oct 15, 2018 at 2:43 PM Clemens Gruber
<clemens.gruber@pqgruber.com> wrote:
>
> Allows signed FIT images to be verified with the public key in the DTB.
> The public key is stored in the bootloader image, which must have been
> verified by the previous stage in the trust chain, before loading it.
>
> Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
> ---
>  boot/uboot/Config.in | 14 ++++++++++++++
>  boot/uboot/uboot.mk  |  5 +++++
>  2 files changed, 19 insertions(+)
>
> diff --git a/boot/uboot/Config.in b/boot/uboot/Config.in
> index 264f343767..620aa02bb9 100644
> --- a/boot/uboot/Config.in
> +++ b/boot/uboot/Config.in
> @@ -460,6 +460,20 @@ config BR2_TARGET_UBOOT_CUSTOM_DTS_PATH
>           To use this device tree source file, the U-Boot configuration
>           file must refer to it.
>
> +config BR2_TARGET_UBOOT_EXT_DTB
> +       bool "External DTB"
> +       help
> +         Put an external DTB in the U-Boot image. Used to store public
> +         keys for verifying signed FIT images.
> +
> +config BR2_TARGET_UBOOT_EXT_DTB_PATH
> +       string "Path to external DTB"
> +       depends on BR2_TARGET_UBOOT_EXT_DTB
> +       help
> +         Path to external DTB to be put in the U-Boot image.
> +         Prepend ${TOPDIR}/ to specify paths relative to the top
> +         buildroot source directory.
> +
>  endif

Would the existing BR2_TARGET_UBOOT_CUSTOM_DTS_PATH option already
allow you to place your custom DTS files?  Then to use them, you would
need to either add a kconfig BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES
fragment to build on your default board kconfig or if you have a
custom board, set the kconfig path in
BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE to your custom kconfig.

I believe the combination KCONFIG values that point at DTS files can
get your  BR2_TARGET_UBOOT_CUSTOM_DTS_PATH dts files included in the
uboot build without setting EXT_DTB.  What do you think?

Matt