dtc: Clean up included Makefile fragments

Currently the Makefile.dtc and Makefile.libfdt fragments include a
number of things that seemed like they might be useful for other
projects embedding the pieces, or for a make dist target.

Well, we have no make dist target, it's become fairly unclear that
these things would actually be useful to embedders (the kernel
certainly doesn't use them), and it's a bunch of stuff with no current
users.

This patch, therefore, removes a bunch of unused definitions from the
Makefile fragments.  It also removes a dependency declared in
Makefile.libfdt (of libfdt.a on the constituent .o files) which was
incorrect (wrong path), and if corrected would be redundant with the
similar dependency in the top-level makefile.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>