verity: optimize for memory usage

Reading the entire image into memory and then blockifying it will
peak at using 2x the size of the image, and hold the size of the
image in memory for the entire computation.  Instead, read
up to 16MB at a time and compute its block hashes before throwing
it away and continuing.

Change-Id: Ibbf1317616765a0dd8de4195b1046a75394c1815
1 file changed