Update VE core submodule to master (6645b10)

6645b10 Merge "Clean up CONTRIBUTING.md and README.md for VE core"
c10a2b5 ve.init.Target: Fix $document property documentation
0bce7f6 Clean up CONTRIBUTING.md and README.md for VE core

Change-Id: Ib24672d322e90b6264f0cf08855d9ce53a7225fa