Make kcfg testfiles valid wrt kcfg.xsd

Jos van den Oever requested to merge work/xsd_fixes into master

The kcfg format is defined by the file kcfg.xsd that is also in this repository. A number of files did not pass validation against kcfg.xsd.

I added a dummy entry to test_emptyentries.kcfg which might not be the best option.

Validity can be checked with:

xmlstarlet val -e -s ./src/kconfig_compiler/kcfg.xsd autotests/kconfig_compiler/*.kcfg

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