Add filter to some boolean variables

add_json_bool converts non-empty string to true, and empty string to
false. But some of boolean variables are meant to be set either "true"
or "false". In that case "false" may lead to a mismatch. This adds
filter to boolean variables which are directly compared to "true", like

    ifeq ($(VAR),true)

Bug: 183483152
Test: build
Change-Id: I0dc4a05293a3233ae4958335781a19e6445a3799