ECMGenerateExportHeader: fix & document CUSTOM_CONTENT_FROM_VARIABLE
While ECMGenerateExportHeader is using CUSTOM_CONTENT_FROM_VARIABLE from GenerateExportHeader itself for adding its own stuff, somehow I had forgotten to also test when offering the same option also with ECMGenerateExportHeader itself ,for consistency. There was just too much focus on all the deprecation macros complexity I guess.
Bug found when proposing to use the flag for kconfigwidgets!156 (merged)
@cgiboudeaux @vkrause @ahmadsamir