Fixing direct import of skeleton_app into latest cfs

Issue as highlighted in https://github.com/nasa/skeleton_app/issues/5
I was able to replicate the issue in Linux and Raspbian
Fixed the files to align to the latest functions and structures of the CFE
This version of skeleton_app can be directly copied into the apps folder of CFS. It will not give any errors when the make command is run.
Also the MID of messages have been updated in "skeleton_app_msgids.h" to avoid conflict with the Message Ids of sample_app which comes with the CFS. There is a high chance that the developer trying to learn might have both sample_app and skeleton_app included. Hence its important to have non-conflicting ids.

nasa / skeleton_app