tools: binman: etype: fit: Append DTB directory to the list of input directories

tools: binman: etype: fit: Append DTB directory to the list of input directories

When specifying a directory containing DTBs with 'fit,fdt-list-dir', it can be
handy not to have to also specify this directory to the input directories of
binman with '-I' option and use the method tools.append_input_dirs() append it.
This avoids to have to specify the DTB directory in both the device tree
provided to binman and through '-I' option to binman.

Signed-off-by: Paul HENRYS <paul.henrys_ext@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>