cmd: pxe_utils: Limit fdtcontroladdr usage to non-fitImage

Commit d5ba6188 ("cmd: pxe_utils: Check fdtcontroladdr in label_boot")
forces '$fdtcontroladdr' DT address as a third parameter of bootm command
even if the PXE transfer pulls in a fitImage which contains configuration
node with its own DT that is preferrable to be passed to Linux. Limit the
$fdtcontroladdr fallback utilization to non-fitImages, since it is highly
likely a fitImage would come with its own DT, while single-file images do
need a separate DT.

Fixes: d5ba6188 ("cmd: pxe_utils: Check fdtcontroladdr in label_boot")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Tested-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peter Hoyes <peter.hoyes@arm.com>
Tested-by: Peter Hoyes <peter.hoyes@arm.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>