dtoc: Allow specifying the base directory for tests

The base directory of U-Boot, where the source is, it currently calculated
from the directory of the dtb_platdata.py script. If this is installed
elsewhere that will not work. Also it is inconvenient for tests.

Add a parameter to allow specifying this base directory.

To test this, pass a temporary directory with some files in it and check
that they are passed to scan_driver().

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