dtoc: Drop use of a local dtb buffer

dtoc: Drop use of a local dtb buffer

At present the Fdt class has its own copy of the device tree. This is
confusing an unnecessary now that pylibfdt has its own. Drop it and
provide access functions to the buffer.

This allows us to move the rest of the implementation to use pylibfdt
methods instead of directly calling libfdt stubs.

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