common: and platform: sections ignored in Addon_config.mk

[bangnoise]

OF release 0.9.8

If you define ADDON_INCLUDES_EXCLUDE in common: and then add to it in a specific directive, it gets ignored, eg

common:
    #exclude for all platforms
    ADDON_INCLUDES_EXCLUDE = libs/mylib/include/never

linux64:
    #exclude for this platform only
    ADDON_INCLUDES_EXCLUDE += libs/mylib/%

doesn't work

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