mvebu: add specific signature support to boot-scr and sdcard-img-ext4

Add image-specific signature support to Build/boot-scr and
Build/sdcard-img-ext4.
These are required to switch root device passed to the kernel on Check
Point V-80.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>

target/linux/mvebu/image/Makefile

@@ -14,7 +14,7 @@ KERNEL_LOADADDR := 0x00008000
 define Build/boot-scr
 	rm -f $@-boot.scr
 	sed \
-		-e 's#@ROOT@#$(IMG_PART_SIGNATURE)#g' \
+		-e 's#@ROOT@#$(if $(1),$(1),$(IMG_PART_SIGNATURE))#g' \
 		-e 's#@DTB@#$(firstword $(DEVICE_DTS))#g' \
 		$(BOOT_SCRIPT).bootscript > $@-new.bootscript
 	mkimage -A arm -O linux -T script -C none -a 0 -e 0 -d $@-new.bootscript $@-boot.scr
@@ -129,7 +129,7 @@ define Build/sdcard-img
 endef
 
 define Build/sdcard-img-ext4
-	SIGNATURE="$(IMG_PART_SIGNATURE)" \
+	SIGNATURE="$(if $(1),$(1),$(IMG_PART_SIGNATURE))" \
 	./gen_mvebu_sdcard_img.sh $@ \
 		$(if $(UBOOT),$(STAGING_DIR_IMAGE)/$(UBOOT)) \
 		83 $(CONFIG_TARGET_KERNEL_PARTSIZE) $@.bootimg \