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>

arch/sandbox/cpu/cpu.c

@@ -340,6 +340,8 @@ void *board_fdt_blob_setup(int *ret)
 	int err;
 	int fd;
 
+	if (gd->fdt_blob)
+		return (void *)gd->fdt_blob;
 	blob = map_sysmem(CONFIG_SYS_FDT_LOAD_ADDR, 0);
 	*ret = 0;
 	if (!state->fdt_fname) {