targets/fcdev3b.h: bring back CONFIG_TARGET_FCDEV3B C preprocessor symbol CONFIG_TARGET_FCMODEM is no longer used anywhere in our code base, thus it is being fully retired. OTOH, CONFIG_TARGET_FCDEV3B is being brought back: with our earlier FC modem family idea now being withdrawn in light of the discovery of Tango modules, FCDEV3B goes back to being its own unique target.