image: push default arch values to arch headers

This pushes the ugly duplicated arch ifdef lists we maintain in various
image related files out to the arch headers themselves.

Acked-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Tested-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/arch/arm/include/asm/u-boot.h b/arch/arm/include/asm/u-boot.h
index ed33327..f30b9fc 100644
--- a/arch/arm/include/asm/u-boot.h
+++ b/arch/arm/include/asm/u-boot.h
@@ -48,4 +48,7 @@
     }			bi_dram[CONFIG_NR_DRAM_BANKS];
 } bd_t;
 
+/* For image.h:image_check_target_arch() */
+#define IH_ARCH_DEFAULT IH_ARCH_ARM
+
 #endif	/* _U_BOOT_H_ */