Added build_flatbuffers function to CMake.

This function is designed to be called by projects that use FlatBuffers.
It takes a list of FlatBuffers schemas and a list of schema include
directories, and generates build rules to create generated headers
and/or binary schemas for those files, as well as optionally copying
all of the text schemas into a directory for your use (for example if
you need to parse text schemas within your app).

See function comments for more details.

Change-Id: I181684b916e91d965e14849d8f83ec2c83e9a4a4