board/common: Add common macro for image generation

Add scaffolding for breaking out image generation to separate make
targets.

board/common/common.mk

@@ -1,3 +1,5 @@
+include $(BR2_EXTERNAL_INFIX_PATH)/board/common/image/image.mk
+
 ifeq ($(TRUSTED_KEYS),y)
 include $(BR2_EXTERNAL_INFIX_PATH)/board/common/uboot/uboot.mk
 

board/common/image/image.mk

@@ -0,0 +1,2 @@
+include $(BR2_EXTERNAL_INFIX_PATH)/board/common/image/ix-image.mk
+include $(sort $(wildcard $(BR2_EXTERNAL_INFIX_PATH)/board/common/image/*/*.mk))

board/common/image/ix-image.mk

@@ -0,0 +1,25 @@
+define inner-ix-image
+
+$(2)_DIR := $$(pkgdir)
+
+$(1): $$($(2)_DEPENDENCIES)
+	@$$(call IXMSG,"$$(if $$($(2)_MESSAGE),$$($(2)_MESSAGE),Creating $(1))")
+	@mkdir -p $$(BUILD_DIR)/$(1)
+	@ \
+	PATH=$$(BR_PATH) \
+	PKGDIR=$$($(2)_DIR) \
+	WORKDIR=$$(BUILD_DIR)/$(1) \
+	BINARIES_DIR=$$(BINARIES_DIR) \
+	ARTIFACT=$$(INFIX_ARTIFACT) \
+	COMPATIBLE=$$(INFIX_COMPATIBLE) \
+	VERSION=$$(INFIX_VERSION) \
+	$$(foreach var,$$($(2)_CONFIG_VARS),$$(var)=$$($(2)_$$(var)) ) \
+	$$($(2)_DIR)/generate.sh $$($(2)_OPTS)
+
+ifeq ($$($(2)),y)
+TARGETS_ROOTFS += $(1)
+endif
+
+endef
+
+ix-image = $(call inner-ix-image,$(pkgname),$(call UPPERCASE,$(pkgname)))