Fix a number of top level doxygen issues.
So that the navigation pane works correctly in browsers.
Some additional doc fixes put in where found (but many more still to go).
Added some dummy .c and .h files to bring the associated docs into line.
makefile changed to allow 'make html' as well as 'make doc' (the latter only does html anyway).
Miscellaneous typos (note particularly lpc43xx gpdma.h, sgpio.h)