binman: Tidy up _SetupDtb() to use its own temporary file

At present EnsureCompiled() uses an file from the 'output' directory (in
the tools module) when compiling the device tree. This is fine in most
cases, allowing useful inspection of the output files from binman.

However in functional tests, _SetupDtb() creates an output directory and
immediately removes it afterwards. This serves no benefit and just
confuses things, since the 'official' output directory is supposed to be
created and destroyed in control.Binman().

Add a new parameter for the optional temporary directory to use, and use a
separate temporary directory in _SetupDtb().

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