bloblist: fix the overriding of fdt from bloblist

bloblist: fix the overriding of fdt from bloblist

When a bloblist is valid and contains fdt, it explicitly means
a previous boot stage is passing transfer list compliant with
Firmware Handoff specification, thus the fdt from bloblist should
not be overridden with the ones from board or env variables.

Fixes: 70fe23859437 ("fdt: Allow the devicetree to come from a bloblist")
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>