Allow entropy to be provided to some operations

generateKey and begin can now optionally take an array of bytes to add
to the rng entropy of the device before the operation. If entropy is
specified and the device does not support add_rng_entropy or the call
fails then that device will not be used, leading to fallback or error
depending on the situation.

Change-Id: Id7d33e3cc959594dfa5483d002993ba35c1fb134