fdt: Makefile: Build python libfdt library if needed

This is needed by binman and dtoc, so if those are being used, check that
the library is present and complain if not. Make sure that any error
appears on stderr so that buildman notices it.

This means that the fallback library (which uses fdtget) will not be used
anymore and swig will need to be installed to use binman / dtoc.

This affects any board which uses binman (currently sunxi and x86) or dtoc
(anything that uses CONFIG_SPL_OF_PLATDATA, currently some rockchip
boards).

Signed-off-by: Simon Glass <sjg@chromium.org>