sandbox: fdt: Avoid overwriting an existing fdt

Since the removal of OF_HOSTFILE logic in board_fdt_blob_setup(), the
logic for obtaining the DT is handled in the OF_BOARD option. If a
devicetree comes from a bloblist it is immediately overwritten by this
function.

Fix this by skipping the function if a devicetree is already present.

This is sort-of a fix for e7fb7896 ("sandbox: Remove OF_HOSTFILE") but
it has only come to light since bloblist was added, so I have not added
a Fixes tag.

Unfortunately it is not possible to report the correct FDT source with
the current code. It might be best to use an error-return code for
board_fdt_blob_setup() so that an error can be reported if the board
does not provide the DT.

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