binman: fit: Refine handling of devicetrees for OF_UPSTREAM

binman: fit: Refine handling of devicetrees for OF_UPSTREAM

With OF_UPSTREAM the dts files are in an SoC-specific subdirectory,
meaning that the resulting dtb files all end up in a similar
subdirectory.

We don't want the subdirectory name to appear as a node name in the FIT,
so handle this as a special case.

Also the default devicetree may have a directory-name prefix, so handle
that when searching through the available devicetree files.

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