doc: doxygen expands tabs to 4 spaces instead of 8

In doxygen, code snippets (and possibly over places) have their
tabulations expanded to 8 spaces which is a bit too much. The TAB_SIZE
configuration parameter let us set the number of spaces, down to 4 which
makes code examples easier to read IMHO.

Change-Id: I62b417496f4e5ec70e80919c28747f1bd44a7bd6