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: e7fb7896 ("sandbox: Remove OF_HOSTFILE")

Signed-off-by: Simon Glass <sjg@chromium.org>