These functions are useful when serializing an unknown number of items
to a blob. The caller can first save the current offset, write a
placeholder uint32, write out (and count) the items, then use
blob_overwrite_uint32 with the saved offset to replace the placeholder
value.

Then, when deserializing, the reader will first read the count and
know how many subsequent items to expect.

(I wrote this code after reading a very similar patch written by
Tapani when he wrote serialization code for IR. Since I re-used the
idea of his code so directly, I've credited him as the author of this
code. --Carl)

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>