Fix #774, do not generate empty include file

[jphickey]

Describe the contribution
Detect the condition where no files were present to fulfill an config include file requirement, and report as an error rather than generating an empty file.

Fixes #774

Testing performed
Create a test config that refers to a nonexistent platform config called "foobar" by setting this in targets.cmake:
SET(TGT1_PLATFORM foobar)

Without this change, make prep still (seemingly) completes successfully but the cfe_platform_cfg.h is actually empty, which results in a later compiler error.

Expected behavior changes
After applying this change, the issue is reported as a make prep error instead, along with a list of files it checked for.

Example:

ERROR: No implementation for cfe_platform_cfg.h found
CMake Error at cmake/global_functions.cmake:119 (message):
  Tested:
  /home/joe/code/cfecfs/github/sample_defs/cfe_platform_cfg.h;/home/joe/code/cfecfs/github/sample_defs/foobar_platform_cfg.h
Call Stack (most recent call first):
  fsw/cfe-core/arch_build.cmake:14 (generate_config_includefile)
  cmake/arch_build.cmake:307 (include)
  CMakeLists.txt:119 (prepare)

No FSW changes - just reporting this configuration error.

System(s) tested on
Ubuntu 20.04

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[jphickey]

Note to reviewers - this patch is against the current IC. It might be worth a fast track/hotfix?

Not a major issue, but avoids a bit of a head scratcher if/when you mis-configure your targets.cmake file.

[CDKnightNASA]

def have run into this personally and with other new users