Add CONFIG flag to use the compiler to generate makefile dependencies

Add CONFIG flag to use the compiler to generate makefile dependencies

Using CONFIG+=GNUmake will enable dependency tracking using included
dependency files, but will use an implicit makefile rule to generate
each .d file.

We now support an additional CONFIG option, gcc_MD_depends, that
instead passes the -MD flag to a GCC (compatible) compiler. This
will generate a .d file as a side-effect of the normal compile
step.

Change-Id: Ib6ce1d93e7f07e316a345bf12aa6f2b4e9a6415d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>