Replace PHONE_CAR_BOARD_CONFIG with more specific PHONE_CAR_BOARD_PRODUCT

The statement
```
include $(PHONE_CAR_BOARD_CONFIG)
```
is difficult to translate to the Starlark configuration file. Fortunately,
all the potential values of this variable have have similar structure, so
```
PHONE_CAR_BOARD_CONFIG := device/google_car/foo/BoardConfig.mk
....
include $(PHONE_CARD_BOARD_CONFIG)
```
can be changed to
```
PHONE_CAR_BOARD_PRODUCT := foo
....
include device/google_car/$(PHONE_CAR_BOARD_PRODUCT)/BoardConfig.mk
```

Bug: 190051051
Test: treehugger
Change-Id: Iaa0797417d43b7c91e1501b41cde89dd04ed502a
diff --git a/blueline/BoardConfig.mk b/blueline/BoardConfig.mk
index b990896..1736bc6 100644
--- a/blueline/BoardConfig.mk
+++ b/blueline/BoardConfig.mk
@@ -14,8 +14,8 @@
 # limitations under the License.
 #
 
-ifdef PHONE_CAR_BOARD_CONFIG
-  include $(PHONE_CAR_BOARD_CONFIG)
+ifdef PHONE_CAR_BOARD_PRODUCT
+  include device/google_car/$(PHONE_CAR_BOARD_PRODUCT)/BoardConfig.mk
 else
   TARGET_BOOTLOADER_BOARD_NAME := blueline
   TARGET_SCREEN_DENSITY := 440
diff --git a/crosshatch/BoardConfig.mk b/crosshatch/BoardConfig.mk
index 828afd9..de92096 100644
--- a/crosshatch/BoardConfig.mk
+++ b/crosshatch/BoardConfig.mk
@@ -14,8 +14,8 @@
 # limitations under the License.
 #
 
-ifdef PHONE_CAR_BOARD_CONFIG
-  include $(PHONE_CAR_BOARD_CONFIG)
+ifdef PHONE_CAR_BOARD_PRODUCT
+  include device/google_car/$(PHONE_CAR_BOARD_PRODUCT)/BoardConfig.mk
 else
   TARGET_BOOTLOADER_BOARD_NAME := crosshatch
   TARGET_SCREEN_DENSITY := 560