build: image: add IMG_ROOTFS and IMG_COMBINED variables

Adding those two variables in order to share them across the tree.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/image.mk

@@ -42,6 +42,8 @@ IMG_PREFIX_VERNUM:=$(if $(CONFIG_VERSION_FILENAMES),$(call sanitize,$(VERSION_NU
 IMG_PREFIX_VERCODE:=$(if $(CONFIG_VERSION_CODE_FILENAMES),$(call sanitize,$(VERSION_CODE))-)
 
 IMG_PREFIX:=$(VERSION_DIST_SANITIZED)-$(IMG_PREFIX_VERNUM)$(IMG_PREFIX_VERCODE)$(IMG_PREFIX_EXTRA)$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))
+IMG_ROOTFS:=$(IMG_PREFIX)-rootfs
+IMG_COMBINED:=$(IMG_PREFIX)-combined
 
 MKFS_DEVTABLE_OPT := -D $(INCLUDE_DIR)/device_table.txt
 
@@ -325,7 +327,7 @@ endif
 
 ifdef CONFIG_TARGET_ROOTFS_CPIOGZ
   define Image/Build/cpiogz
-	( cd $(TARGET_DIR); find . | cpio -o -H newc -R root:root | gzip -9n >$(BIN_DIR)/$(IMG_PREFIX)-rootfs.cpio.gz )
+	( cd $(TARGET_DIR); find . | cpio -o -H newc -R root:root | gzip -9n >$(BIN_DIR)/$(IMG_ROOTFS).cpio.gz )
   endef
 endif