security: Rename KeyChain::signInTpm to KeyChain::signDataInTpm, and make it priviate.

This method is only used internally by KeyChain, and its name conflicts with a virtual method of Tpm.

Change-Id: I9f2f75d8451f93ba5264f6108a708a01a4d354eb
node: Fix of getting a pointer to the static member function (& necessary for some compilers)

Change-Id: I7b48310f37a189ee325c1cf9460010387df2400c