image: Split FIT code into new image-fit.c

The FIT code is about half the size of the >3000-line image.c. Split this
code into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

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_FIT) += image-fit.o
 COBJS-y += memsize.o
 COBJS-y += stdio.o