DFU: img util: Fixed warnings when compiling with newlib

Fixed format error when compiling with gcc and newlib.
Used standard formats (%u) instead of inttypes formats (PRIxxx)
since Zephyr redefines the standard formats, that way it should always
be in line with the Zephyr types.

Compiled with and without newlib using gcc.

Signed-off-by: Dennis Wildmark <dennis.wildmark@assaabloy.com>