Merge pull request nasa#907 from jphickey/fix-906-cmake-system

Fix nasa#906, Update variable checks in read_targetconfig

Author: yammajamma

cmake/global_functions.cmake

@@ -185,10 +185,10 @@ function(read_targetconfig)
   endwhile()
 
   foreach(CPUNAME ${MISSION_CPUNAMES})
-    if (DEFINED SIMULATION)
+    if (SIMULATION)
       # if simulation use simulation system architecture for all targets
       set(TOOLCHAIN_NAME "${SIMULATION}")
-    elseif (DEFINED ${CPUNAME}_SYSTEM)
+    elseif (${CPUNAME}_SYSTEM)
       # get the target system arch identifier string
       set(TOOLCHAIN_NAME "${${CPUNAME}_SYSTEM}")
     else()