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: Icf47e7ca0bf01d6b3ee6d86e144460a41a277939