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.
Remove tex header templates only used in latex document generation.
Clarify the HACKING document to reflect no pdf/latex
Fixes: 9443856b doc: drop latex support