image: Split host code out into its own file

To avoid having #ifdefs in a few functions which are completely different
in the board and host code, create a new image-host.c file.

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

tools/Makefile

@@ -113,6 +113,7 @@ dumpimage-mkimage-objs := aisimage.o \
 			lib/fdtdec_common.o \
 			lib/fdtdec.o \
 			common/image.o \
+			common/image-host.o \
 			imagetool.o \
 			imximage.o \
 			imx8image.o \