loader: Fix dtb loading

When calling file_findfile with only a type it returns
the first file matching the type. But in fdt_apply_overlays we
then iterate on the next files and try loading them as dtb overlays.
Fix this by checking the type one more time.

Reported by: Mark Millard <marklmi@yahoo.com>

(cherry picked from commit b464c459eaf4e63a3abaaa1ecf51c9d6e9f7521e)