[new uImage] Correct raw FDT blob handlig when CONFIG_FIT is disabled

Dual format image code must properly handle all three FDT passing methods:
- raw FDT blob passed
- FDT blob embedded in the legacy uImage
- FDT blob embedded in the new uImage

This patch enables proper raw FDT handling when no FIT imaeg support
is compiled in. This is a bit tricky as we must dected FIT format even
when FIT uImage handling is not enabled as both FIT uImages and raw FDT
blobs use tha same low level format (libfdt).

Signed-off-by: Marian Balakowicz <m8@semihalf.com>