McuSupportPlugin: reduce complexity of json kit files

This was originally task https://bugreports.qt.io/browse/UL-2993 , adapted to fit within the Epic for refactoring the McuSupport QtCreator Plugin.

The kit json data contains arrays for specifying "pathEntries", "environmentEntries" and "cmakeEntries", as part of "platform", "boardSdk" and "freeRtos".

This complexity can be reduced to a single array "packageEntries". A packageEntry would directly map to the McuPackage structure within the plugin code.  One package entry includes information about:

• cmake variable name (if it applies)
• environment variable name (if it applies)
• add to system path flag (false by default)