Calculate count for customized dd blocksize

In previous commit we introduce a configurable blocksize option for
'dd' command, however the 'count' value wasn't updated accordingly.
As a result, count could be too small which is bug.  This patch
introduce a private method to 1) validate custom dd blocksize; 2)
calculate correct count value.

Fix bug: 1192258

Change-Id: Icfd2d80e8f615ed585bd1d40c741a97cf55299d3
Merge "Calculate count for customized dd blocksize"