Disallow 4:1/1:4 partitions for BLOCK_128x128

The change can be temporarily reverted by setting ALLOW_128X32_BLOCKS
to 1 (for easy "is this better?" testing: we'll get rid of the option
once it's clear what we're doing).

Since this means we have to modify the "num_partition_types"
calculation in functions like read_partition, we move it into a helper
function called partition_cdf_length(). Doing this makes the bodies of
read_partition and write_partition somewhat simpler than they were.

Change-Id: I3573f62f71dc3344aed2dcb3e423e034eb56a7c5