image: Split libfdt code into image-fdt.c

The image file is still very large, and some of the code is only used when
libfdt is in use. Move this code into a new file.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/Makefile

@@ -231,6 +231,7 @@ COBJS-$(CONFIG_BOUNCE_BUFFER) += bouncebuf.o
 COBJS-y += console.o
 COBJS-y += dlmalloc.o
 COBJS-y += image.o
+COBJS-$(CONFIG_OF_LIBFDT) += image-fdt.o
 COBJS-$(CONFIG_FIT) += image-fit.o
 COBJS-y += memsize.o
 COBJS-y += stdio.o