It's calculated later in our mkuserimg.sh.
BOARD_EGL_CFG \
BOARD_BOOTIMAGE_PARTITION_SIZE \
BOARD_RECOVERYIMAGE_PARTITION_SIZE \
BOARD_EGL_CFG \
BOARD_BOOTIMAGE_PARTITION_SIZE \
BOARD_RECOVERYIMAGE_PARTITION_SIZE \
- BOARD_SYSTEMIMAGE_PARTITION_SIZE \
BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE \
BOARD_USERDATAIMAGE_FILE_SYSTEM_TYPE \
BOARD_USERDATAIMAGE_PARTITION_SIZE \
BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE \
BOARD_USERDATAIMAGE_FILE_SYSTEM_TYPE \
BOARD_USERDATAIMAGE_PARTITION_SIZE \