stm32mp: stm32prog: change one message level to debug

Move the message "Invalid or missing layout file."
to debug level as it is a normal behavior and not an error
and add the missing '\n'.

This patch avoids the strange trace :
  Boot over usb0!
  Invalid or missing layout file.DFU alt info setting: done

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>