extract_dts_includes.py has been generating DT output and then
concatenating it with fixup header files to create
'generated_dts_board.h'.
In this patch we instead introduce a source file named
'generated_dts_board.h' and have it \#include the appropriate DT
output and fixup files.
This results in a simpler system because users can now read
'generated_dts_board.h' as C source code to see how fixup files and
generated DT output relate to each other. Whereas before they would
have to either read documentation or python code to gain the same
understanding.
Also, it reduces the scope and complexity of one of our most bloated
python scripts, extract_dts_includes.py.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>