liboverlay: explicitly set alignment to 1M for secure buffers

On systems that do not have a Contiguous Memory Allocator (CMA) it is
the responsibily of the client to specify the required address alignment.

Additionally, we centralize size alignment to so that overlay clients
do not need to take of this.

Change-Id: Id6cec19bf59826fca2617c856bb62968790bf71c
3 files changed