doc: restructure application primer

Merge build system and application primer into one and put
all in one file to make it easier to follow.

Jira: ZEP-686
Change-Id: I64ec7ef7a6aa2ad80496ca0ee3ddc3d7fe09a5d7
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
14 files changed