Move combo makefile parsing

These files may be setting variables usually defined in the
BoardConfig. This moves that setting into board_config.mk so
that the relevant variables can be made readonly.

Test: build_test
Test: diff presubmit target_files vs base cl
Change-Id: I4dab09fa67c0f2e88a8ce1402a35b4ef5f8a0560
diff --git a/core/board_config.mk b/core/board_config.mk
index 528c133..93e07a0 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -89,6 +89,16 @@
 
 INTERNAL_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE))
 
+# The combo makefiles sanity-check and set defaults for various CPU configuration
+combo_target := TARGET_
+combo_2nd_arch_prefix :=
+include $(BUILD_SYSTEM)/combo/select.mk
+
+ifdef TARGET_2ND_ARCH
+  combo_2nd_arch_prefix := $(TARGET_2ND_ARCH_VAR_PREFIX)
+  include $(BUILD_SYSTEM)/combo/select.mk
+endif
+
 ifeq ($(TARGET_CPU_ABI),)
   $(error No TARGET_CPU_ABI defined by board config: $(board_config_mk))
 endif