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