GHS: Allow setting GHS variables from toolchain file

* Update generator to use the current variable definition,
  not the CACHE definition.
* Avoid overwriting toolchain variables and avoid developer warnings
  when setting up the GHS variables.
  -- A cache entry is only required when:
     a) The toolchain uses set( CACHE ) to set the variable
     b) A -D or preset value was used to set the variable

     The cache entry is required so that the TYPE gets set properly.

  -- Avoid the Policy CMP0126 warnings:
     setting cache variable when normal variable exists
* Move GHS_PRIMARY_TARGET back into area so non-GHS generator
  toolchain file can trigger the OS search.