fdt: Make OF_BOARD a bool option

This should not be a separate option from OF_SEPARATE. It is a run-time
option to override the devicetree, even if present.

Move the option out of the choice.

Disable BINMAN_FDT for a few boards which don't actually use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/qemu-ppce500_defconfig b/configs/qemu-ppce500_defconfig
index ae4cd8b..81d12d5 100644
--- a/configs/qemu-ppce500_defconfig
+++ b/configs/qemu-ppce500_defconfig
@@ -56,4 +56,5 @@
 CONFIG_VIRTIO_NET=y
 CONFIG_VIRTIO_BLK=y
 CONFIG_ADDR_MAP=y
+# CONFIG_BINMAN_FDT is not set
 CONFIG_PANIC_HANG=y
diff --git a/configs/qemu-riscv32_spl_defconfig b/configs/qemu-riscv32_spl_defconfig
index c442296..8889b02 100644
--- a/configs/qemu-riscv32_spl_defconfig
+++ b/configs/qemu-riscv32_spl_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL=y
 CONFIG_TARGET_QEMU_VIRT=y
 CONFIG_RISCV_SMODE=y
+# CONFIG_OF_BOARD_FIXUP is not set
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_LOAD_ADDR=0x80200000
 CONFIG_FIT=y
@@ -18,3 +19,4 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM_MTD=y
 CONFIG_SYSRESET_SBI=y
+# CONFIG_BINMAN_FDT is not set
diff --git a/configs/qemu-riscv64_spl_defconfig b/configs/qemu-riscv64_spl_defconfig
index e02ca3a..8f87594 100644
--- a/configs/qemu-riscv64_spl_defconfig
+++ b/configs/qemu-riscv64_spl_defconfig
@@ -19,3 +19,4 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM_MTD=y
 CONFIG_SYSRESET_SBI=y
+# CONFIG_BINMAN_FDT is not set
diff --git a/dts/Kconfig b/dts/Kconfig
index b7c4a2f..9994ab1 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -97,11 +97,9 @@
 choice
 	prompt "Provider of DTB for DT control"
 	depends on OF_CONTROL
-	default OF_BOARD if SANDBOX
 
 config OF_SEPARATE
 	bool "Separate DTB for DT control"
-	depends on !SANDBOX
 	help
 	  If this option is enabled, the device tree will be built and
 	  placed as a separate u-boot.dtb file alongside the U-Boot image.
@@ -114,14 +112,17 @@
 	  and development only and is not recommended for production devices.
 	  Boards in the mainline U-Boot tree should not use it.
 
+endchoice
+
 config OF_BOARD
 	bool "Provided by the board (e.g a previous loader) at runtime"
+	default y if SANDBOX
 	help
 	  If this option is enabled, the device tree will be provided by
-	  the board at runtime if the board supports it, instead of being
-	  bundled with the image.
+	  the board at runtime if the board supports it. The device tree bundled
+	  with the image (if any) will be overridden / ignored.
 
-endchoice
+	  A device tree file must be provided in the tree.
 
 config DEFAULT_DEVICE_TREE
 	string "Default Device Tree for DT control"