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-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