Fix nasa#906, Update variable checks in read_targetconfig

This was using "DEFINED" to check if these variables were set.
Problem discovered is that this is always true because "SIMULATION"
is a cache var set from an environment variable, so it ALWAYS defined,
it is just empty if not being used.

Fix is to use if (SIMULATION) rather than if (DEFINED SIMULATION) which
should only be true if the string is not empty, as intended.

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()