Refactor the responder functions for file I/O

The ReadFileIntoMemory and WriteFileFromMemory command's responder
function is refactored to return the PLDM response message. The caller
cannot know the space for the response buffer if the responder has a
variable length of response.

Change-Id: I81f649098a7711772b35ae3d0d8d23c7cd24e7e7
Signed-off-by: Tom Joseph <tomjoseph@in.ibm.com>
3 files changed