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: e7fb789612 ("sandbox: Remove OF_HOSTFILE")
Signed-off-by: Simon Glass <sjg@chromium.org>