build_image.py recognize BOARD_VENDORIMAGE_PARTITION_RESERVED_SIZE

Test: `make vendorimage`

Test: `make vendorimage` with the following:
    - install a large file to vendor image
    fails as expected (because _PARTITION_SIZE is exceeded)

Test: `make vendorimage` with the following:
    - set PRODUCT_USE_LOGICAL_PARTITIONS to true
    - set BOARD_VENDORIMAGE_PARTITION_RESERVED_SIZE
    fails as expected (BOARD_VENDORIMAGE_PARTITION_SIZE needs
    to be undefined)

Test: `make vendorimage` with the following:
    - install a large file to vendor image
    - set PRODUCT_USE_LOGICAL_PARTIIONS to true
    - add a small BOARD_VENDORIMAGE_PARTITION_RESERVED_SIZE
    - remove BOARD_VENDORIMAGE_PARTITION_SIZE
    build succeeds.

Bug: 79106666

Change-Id: Ica8fdce64e8f37d91e66e3d49c0c74fadd15a832