fb: add config.xml option to permanently force permanent menu key

devices like m7 are a kind of hybrid between navbar and hw key devices and
need a menu button in applications although it is categorized as a hw key
device. Give an option to permanently enable the menu key via overlay

            PS1:  Moved symbols to symbols_custom

Change-Id: I926b1b864e544c51fb225918022730ccab3698b6