Implement discovery commands

This commit does the following
- Implements the GetPLDMTypes and GetPLDMCommands commands. These are
  commands that need to be handled by a PLDM device as part of the
  initial PLDM discovery.
- Sets up the build infrastructure: separate libraries for PLDM
  encode/decode libs and the PLDM responder.

Change-Id: I65fa222d2a681c473f579c8e30d84faaf94fe754
Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>