Export some board config vars to autogenerate android_info.prop

BoardInfoFiles will be
- TARGET_BOARD_INFO_FILES, else
- TARGET_BOARD_INFO_FILE, else
- $(TARGET_DEVICE_DIR)/board-info.txt, if one exists

Bug: 375500423
Test: verified that /vendor/build.prop is same between kati and soong
for oriole (modulo timestamp diffs)

Change-Id: Ibf45989a5a8b3a1e25eb1f9ad5281b640094d800