build: Move inclusion of BoardConfigExtra.mk to board_config.mk

Some kind of extras needs to alter variables that becomes read-only
after inclusion of board_config.mk, or wants some variables getting
processed by board_config.mk. For example, native bridge.

Change-Id: If7ef0dcfc30b52940017662fa7402e4970dbc3c7

core/board_config.mk

@@ -240,6 +240,7 @@ endif
 $(call dump-phase-start,BOARD,,,, build/make/core/board_config.mk)
 ifndef RBC_PRODUCT_CONFIG
 include $(board_config_mk)
+-include vendor/extra/BoardConfigExtra.mk
 else
   $(shell mkdir -p $(OUT_DIR)/rbc)
   $(call dump-variables-rbc, $(OUT_DIR)/rbc/make_vars_pre_board_config.mk)

core/config.mk

@@ -453,7 +453,6 @@ endif
 # See envsetup.mk for a description of SCAN_EXCLUDE_DIRS
 FIND_LEAVES_EXCLUDES := $(addprefix --prune=, $(SCAN_EXCLUDE_DIRS) .repo .git)
 
--include vendor/extra/BoardConfigExtra.mk
 ifneq ($(LINEAGE_BUILD),)
 include vendor/lineage/config/BoardConfigLineage.mk
 endif