image: Split board code out into its own file

To avoid a large #ifdef in the image.c file, move the affected code into
a separate file.

Avoid any style fix-ups for easier review. Those are in the next patch.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index fb8173a..e783902 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -101,7 +101,7 @@
 endif
 endif
 
-obj-y += image.o
+obj-y += image.o image-board.o
 obj-$(CONFIG_$(SPL_TPL_)HASH) += hash.o
 obj-$(CONFIG_ANDROID_AB) += android_ab.o
 obj-$(CONFIG_ANDROID_BOOT_IMAGE) += image-android.o image-android-dt.o