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>

board/emulation/common/Kconfig

@@ -13,3 +13,15 @@ config MTDPARTS_NOR1
 	help
 	  This define the partition of nor1 used to build mtparts dynamically
 	  for the u-boot env stored on nor1.
+
+config QEMU_MANUAL_DTB
+	bool "Manually provide a device tree to QEMU"
+	help
+	  For some use cases, such as FIT validation where a public key must be
+	  placed in U-Boot's device tree, we need to override the device tree
+	  that QEMU would normally provide to us.
+
+	  Note: this work-around is necessary since Linaro has blocked
+	  addition of a feature to support additions to the QEMU devicetree:
+
+	    Link: https://patchwork.kernel.org/project/qemu-devel/patch/20210926183410.256484-1-sjg@chromium.org/#24481799