emulation: fdt: Allow using U-Boot's device tree with QEMU

At present it is impossible to change the qemu_arm64 defconfig to
obtain a devicetree from the U-Boot build.

This is necessary for FIT validation, for example, where the signature
node must be compiled into U-Boot.

A proposed change to QEMU to allow device tree additions has been
blocked for several years. The only known workaround is to use QEMU's
dumpdtb option, merge in the signature node manually, disable
OF_HAS_PRIOR_STAGE and then start QEMU with special arguments. This is
complicated enough that it is documented in U-Boot[1].

Unfortunately the only way to disable OF_HAS_PRIOR_STAGE at present is
to hack the Kconfig.

Add a new QEMU_MANUAL_DTB Kconfig option which makes OF_HAS_PRIOR_STAGE
optional, thus avoiding needing to patch U-Boot to get this working.

This seems a clearer solution than just making OF_HAS_PRIOR_STAGE
visible, since that symbol is intended to be set automatically by each
platform.

Series-to: u-boot
Series-cc: trini
Series-cc: Peter Maydell <peter.maydell@linaro.org>
Series-cc: Andrew Phelps <andrew.phelps@canonical.com>
Series-cc: ilias
Series-changes: 2
- Add a new QEMU-specific Kconfig instead
- Move patch into the standard-passage series

Series-changes: 3
- Fix 'usiing' typo
- Add mention of QEMU_MANUAL_DTB in doc/

[1] https://docs.u-boot.org/en/latest/develop/devicetree/dt_qemu.html
Link: https://patchwork.kernel.org/project/qemu-devel/patch/20210926183410.256484-1-sjg@chromium.org/#24481799

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>

dts/Kconfig

@@ -205,8 +205,13 @@ config OF_HAS_PRIOR_STAGE
 	  development purposes, but it is not recommended, and likely will not
 	  even work, for production systems.
 
-	  Note: This option must be set in Kconfig and cannot be enabled or
-	  disabled in the board's defconfig file.
+	  Note: This option can be disabled for QEMU using QEMU_MANUAL_DTB thus
+	  allowing QEMU to support FIT validation, where the devicetree must
+	  include a public key.
+
+	  See also this for context:
+
+	  https://patchwork.kernel.org/project/qemu-devel/patch/20210926183410.256484-1-sjg@chromium.org/#24481799
 
 config OF_OMIT_DTB
 	bool "Omit the device tree output when building"