dtoc: Allow providing a directory to write files to

dtoc: Allow providing a directory to write files to

At present dtoc writes only a single file on each invocation. U-Boot
writes the two files it needs by separate invocations of dtoc. Since dtoc
now scans all U-Boot driver source, this is fairly slow (about 1 second
per file).

It would be better if dtoc could write all the files at once.

In preparation for this, add a way to specify an output directory for the
files.

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