sandbox: Correct SPL condition for building devicetree

With sandbox, CONFIG_SANDBOX is y so the current rule ends up building
the devicetree for only those SPL builds where it is unwanted.

Correct the condition. This allows sandbox_vpl to produce a
u-boot-vpl.dtb file.

Fixes: e7fb789612e ("sandbox: Remove OF_HOSTFILE")

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index e450ffd..407fc52 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -314,7 +314,7 @@
 #   - we have either OF_SEPARATE or OF_HOSTFILE
 build_dtb :=
 ifneq ($(CONFIG_$(SPL_TPL_)OF_REAL),)
-ifeq ($(CONFIG_OF_SEPARATE)$(CONFIG_SANDBOX),y)
+ifneq ($(CONFIG_OF_SEPARATE)$(CONFIG_SANDBOX),)
 build_dtb := y
 endif
 endif