index file changes triggers new model test

[werner2101]

When a single index section is updated then all tests of an index file became invalid, as all model tests depend on the index files.

The test should only depend on the device section.

Possible solutions:

1. split the index files into sections with a custom scons builder
2. create a scons Decider function (see scons Documentation)
3. remove index file as source and just add a hash-value of the index section as environment (not sure if this is an allowed dependancy)

(found by Felix)