Generate doc root doxygenlayout file, as well as devices stuff, based on main Makefile $(TARGETS) and template files. Avoids painfull sync/merge of 20 files+ when adding a new device. bonus : allow to build only one device doc easily (make TARGETS=stm32f0 doc) regression: we currently loose device "fancy" naming as device name is guessed (toUpper()..) from folder name.