Allow finer control over how product variables are inherited.

Change-Id: I3abc22eea94293d1d0ebf0a81b396ebea0baf5a8
(cherry picked from commit 29357f5)

core/product.mk

@@ -141,26 +141,72 @@ $(foreach p,$(PRODUCTS),$(call dump-product,$(p)))
 endef
 
 #
-# $(1): product to inherit
+# Internal function. Appends inherited product variables to an existing one.
 #
-# Does three things:
-#  1. Inherits all of the variables from $1.
-#  2. Records the inheritance in the .INHERITS_FROM variable
-#  3. Records that we've visited this node, in ALL_PRODUCTS
+# $(1): Product variable to operate on
+# $(2): Value to append
 #
-define inherit-product
+define inherit-product_append-var
+  $(if $(findstring ../,$(2)),\
+    $(eval np := $(call normalize-paths,$(2))),\
+    $(eval np := $(strip $(2))))\
+  $(eval $(1) := $($(1)) $(INHERIT_TAG)$(np))
+endef
+
+#
+# Internal function. Prepends inherited product variables to an existing one.
+#
+# $(1): Product variable to operate on
+# $(2): Value to prepend
+#
+define inherit-product_prepend-var
+  $(eval $(1) := $(INHERIT_TAG)$(strip $(2)) $($(1)))
+endef
+
+#
+# Internal function. Tracks visited notes during inheritance resolution.
+#
+# $(1): Product being inherited
+#
+define inherit-product_track-node
   $(if $(findstring ../,$(1)),\
     $(eval np := $(call normalize-paths,$(1))),\
     $(eval np := $(strip $(1))))\
-  $(foreach v,$(_product_var_list), \
-      $(eval $(v) := $($(v)) $(INHERIT_TAG)$(np))) \
   $(eval inherit_var := \
       PRODUCTS.$(strip $(word 1,$(_include_stack))).INHERITS_FROM) \
   $(eval $(inherit_var) := $(sort $($(inherit_var)) $(np))) \
   $(eval inherit_var:=) \
   $(eval ALL_PRODUCTS := $(sort $(ALL_PRODUCTS) $(word 1,$(_include_stack))))
 endef
 
+#
+# $(1): product to inherit
+#
+# Does three things:
+#  1. Inherits all of the variables from $1, prioritizing existing settings.
+#  2. Records the inheritance in the .INHERITS_FROM variable
+#  3. Records that we've visited this node, in ALL_PRODUCTS
+#
+
+define inherit-product
+  $(foreach v,$(_product_var_list), \
+      $(call inherit-product_append-var,$(v),$(1))) \
+  $(call inherit-product_track-node,$(1))
+endef
+
+#
+# $(1): product to inherit
+#
+# Does three things:
+#  1. Inherits all of the variables from $1, prioritizing inherited settings.
+#  2. Records the inheritance in the .INHERITS_FROM variable
+#  3. Records that we've visited this node, in ALL_PRODUCTS
+#
+define prepend-product
+  $(foreach v,$(_product_var_list), \
+      $(call inherit-product_prepend-var,$(v),$(1))) \
+  $(call inherit-product_track-node,$(1))
+endef
 
 #
 # Do inherit-product only if $(1) exists
@@ -169,6 +215,13 @@ define inherit-product-if-exists
   $(if $(wildcard $(1)),$(call inherit-product,$(1)),)
 endef
 
+#
+# Do inherit-product-prepend only if $(1) exists
+#
+define prepend-product-if-exists
+  $(if $(wildcard $(1)),$(call prepend-product,$(1)),)
+endef
+
 #
 # $(1): product makefile list
 #